Fig. 113.—Ganglia of the Cranial Nerves of an Ammocœtes, 4 mm. in length, projected on to the Median Plane.(AfterKupffer.)A-B, the line of epibranchial ganglia;au., auditory capsule;nc., notochord;Hy., tube of hypophysis;Or., oral cavity;u.l., upper lip;l.l.lower lip;vel., septum between oral and respiratory cavities;V.,VII.,IX.,X., cranial nerves;x., nerve with four epibranchial ganglia.
Fig. 113.—Ganglia of the Cranial Nerves of an Ammocœtes, 4 mm. in length, projected on to the Median Plane.(AfterKupffer.)A-B, the line of epibranchial ganglia;au., auditory capsule;nc., notochord;Hy., tube of hypophysis;Or., oral cavity;u.l., upper lip;l.l.lower lip;vel., septum between oral and respiratory cavities;V.,VII.,IX.,X., cranial nerves;x., nerve with four epibranchial ganglia.
Fig. 113.—Ganglia of the Cranial Nerves of an Ammocœtes, 4 mm. in length, projected on to the Median Plane.(AfterKupffer.)
A-B, the line of epibranchial ganglia;au., auditory capsule;nc., notochord;Hy., tube of hypophysis;Or., oral cavity;u.l., upper lip;l.l.lower lip;vel., septum between oral and respiratory cavities;V.,VII.,IX.,X., cranial nerves;x., nerve with four epibranchial ganglia.
In accordance with the views put forward in this book, a possible interpretation of these plakodal ganglia would be given as follows:—
Beard, who, after Froriep, drew attention to this relation of the cranial ganglia to special skin-patches, has compared them with the parapodial ganglia of annelids,i.e. ganglia in connection with annelidan appendages; whether we are here obtaining a glimpse of the far-off annelidan ancestry of both arthropods and vertebrates it would be premature at present to say. It is natural enough to expect, on my view, to find evidence of annelidan ancestry invertebrate embryology (as has been so often asserted to be the case), seeing that undoubtedly the Arthropoda are an advanced stage of Annelida; and, indeed, the way is not a long one when we consider Beecher's evidence that the Trilobita belong to the Phyllopoda, certainly a primitive crustacean group, which Bernard derives directly from the annelid group Chætopoda. If, then, these plakodal ganglia indicate the former presence of appendages, we obtain this result:—The foremost ganglion on each side possesses one plakodal ganglion, and therefore indicates an anterior pair of appendages, possibly the cheliceræ. Then comes the peculiar nerve with four plakodal ganglia indicating on each side four appendages close together, possibly the endognaths. Then, finally, on each side, the second large ganglion with two plakodal ganglia, indicating two pairs of appendages, possibly the ectognaths and the metastoma.
Summary.
The consideration of the history of the cranial segmentation shows that whereas, from the commencement of that history, the evidence for two ventral segments supplied by the trigeminal nerve is clear and unmistakable, later observers have tended more and more to increase the number of these segments, until at the present time the evidence is in favour of at least six, probably seven, as the number of segments supplied by the motor part of the trigeminal.So, also, the original evidence for the number of dorsal or somatic segments limits the number to three, innervated respectively by the oculomotor (III.), trochlear (IV.), and abducens (VI.) nerves, or rather two, since the last nerve belongs to the facial segment. The muscles which these three nerves supply are derived respectively from the walls of the premandibular, mandibular, and hyoid cœlomic cavities.Later evidence points strongly to the conclusion that the oculomotor nerve and the premandibular cavity represent not one segment but the fusion of four, while the mandibular cavity represents two segments. In addition to these, Miss Platt has discovered a still more anterior head-cavity, which she has named the anterior cavity, so that the pro-otic segments on this reckoning are seven in number, viz.: (1) the anterior cavity, (2, 3, 4, 5) the premandibular cavity, (6, 7) the mandibular cavity. The somatic muscles belonging to these dorsal segments are the eye-muscles, which are all dorso-ventral in position, and are not the same as the longitudinal somatic muscles, but belong to a distinct dorso-ventral segmental group, the only representative of which at present known in the mesosomatic region is the external rectus innervated by the VIth nerve.These head-cavities, and these muscles of the vertebrate, resemble the corresponding cavities and muscles of the invertebrate to an extraordinarydegree, so that it becomes easy to see how the dorso-ventral muscles of the prosomatic segments of the latter have become converted into the eye-musculature of the former. The most powerful proof of all that such a conversion has taken place is that a natural and simple explanation is at once given of the extraordinary course taken by the IVth or trochlear nerve. Ever since neurology began, the course of this nerve has arrested the attention of anatomists. Why should just this one pair of nerve-roots of all those in the whole body be directed dorsalwards instead of ventralwards, and cross each other in the valve of Vieussens, each to supply a simple eye-muscle (the superior oblique) belonging to the other side? For generations anatomists have wondered and found no solution, and yet, without any straining of hypotheses, in consequence simply of the investigation of the anatomy of the corresponding pair of muscles in the scorpion group, the solution is immediately apparent.This pair of muscles alone, of all the musculature attached to the carapace, crosses the mid-dorsal line to be attached to the other side, thus carrying its nerve with it to the other side; by a continuation of the same process the relation of the trochlear to the superior oblique muscle can be explained.The comparison of the eye-muscles of the vertebrate with the dorso-ventral segmented muscles of the invertebrate makes the number and nature of the pro-otic segments much clearer.
The consideration of the history of the cranial segmentation shows that whereas, from the commencement of that history, the evidence for two ventral segments supplied by the trigeminal nerve is clear and unmistakable, later observers have tended more and more to increase the number of these segments, until at the present time the evidence is in favour of at least six, probably seven, as the number of segments supplied by the motor part of the trigeminal.
So, also, the original evidence for the number of dorsal or somatic segments limits the number to three, innervated respectively by the oculomotor (III.), trochlear (IV.), and abducens (VI.) nerves, or rather two, since the last nerve belongs to the facial segment. The muscles which these three nerves supply are derived respectively from the walls of the premandibular, mandibular, and hyoid cœlomic cavities.
Later evidence points strongly to the conclusion that the oculomotor nerve and the premandibular cavity represent not one segment but the fusion of four, while the mandibular cavity represents two segments. In addition to these, Miss Platt has discovered a still more anterior head-cavity, which she has named the anterior cavity, so that the pro-otic segments on this reckoning are seven in number, viz.: (1) the anterior cavity, (2, 3, 4, 5) the premandibular cavity, (6, 7) the mandibular cavity. The somatic muscles belonging to these dorsal segments are the eye-muscles, which are all dorso-ventral in position, and are not the same as the longitudinal somatic muscles, but belong to a distinct dorso-ventral segmental group, the only representative of which at present known in the mesosomatic region is the external rectus innervated by the VIth nerve.
These head-cavities, and these muscles of the vertebrate, resemble the corresponding cavities and muscles of the invertebrate to an extraordinarydegree, so that it becomes easy to see how the dorso-ventral muscles of the prosomatic segments of the latter have become converted into the eye-musculature of the former. The most powerful proof of all that such a conversion has taken place is that a natural and simple explanation is at once given of the extraordinary course taken by the IVth or trochlear nerve. Ever since neurology began, the course of this nerve has arrested the attention of anatomists. Why should just this one pair of nerve-roots of all those in the whole body be directed dorsalwards instead of ventralwards, and cross each other in the valve of Vieussens, each to supply a simple eye-muscle (the superior oblique) belonging to the other side? For generations anatomists have wondered and found no solution, and yet, without any straining of hypotheses, in consequence simply of the investigation of the anatomy of the corresponding pair of muscles in the scorpion group, the solution is immediately apparent.
This pair of muscles alone, of all the musculature attached to the carapace, crosses the mid-dorsal line to be attached to the other side, thus carrying its nerve with it to the other side; by a continuation of the same process the relation of the trochlear to the superior oblique muscle can be explained.
The comparison of the eye-muscles of the vertebrate with the dorso-ventral segmented muscles of the invertebrate makes the number and nature of the pro-otic segments much clearer.
CHAPTER IX
THE PROSOMATIC SEGMENTS OF AMMOCŒTES
The prosomatic region in Ammocœtes.—The suctorial apparatus of the adult Petromyzon.—Its origin in Ammocœtes.—Its derivation from appendages.—The segment of the lower lip or metastomal segment.—The tentacular segments.—The tubular muscles.—Their segmental arrangement.—Their peculiar innervation.—Their correspondence with the system of veno-pericardial muscles in Limulus.—The old mouth or palæostoma.—The pituitary gland.—Its comparison with the coxal gland of Limulus.—Summary.
The prosomatic region in Ammocœtes.—The suctorial apparatus of the adult Petromyzon.—Its origin in Ammocœtes.—Its derivation from appendages.—The segment of the lower lip or metastomal segment.—The tentacular segments.—The tubular muscles.—Their segmental arrangement.—Their peculiar innervation.—Their correspondence with the system of veno-pericardial muscles in Limulus.—The old mouth or palæostoma.—The pituitary gland.—Its comparison with the coxal gland of Limulus.—Summary.
In the last chapter it was seen not to be incompatible with both the anatomical and morphological evidence to look upon the trigeminal nerves as having originally supplied the seven prosomatic pairs of appendages of the invertebrate ancestor, the foremost of which, the cheliceræ, and the four pairs of endognaths dwindled away and became insignificant, leaving as trace of their former presence the descending root of the Vth nerve; while the two hindmost pairs, the ectognaths and the chilaria, or metastoma, remained vigorous and developed, leaving as proof of their presence thenucleus masticatorius. Evidence in favour of this suggestion and of the nature of the dwindling process is afforded when we examine what the trigeminus does supply in Ammocœtes. In all vertebrates this nerve supplies the great muscles of mastication which, in all gnathostomatous fishes, move the jaws. The lowest fishes, the cyclostomes, possess no jaws; they take in their food by attaching themselves to their prey and by means of rasping teeth situated in serried rows within the circular mouth, combined with a powerful suctorial apparatus, they suck the juices of the fish they feed upon. Not possessing jaws, they feed by suction on the living animal, a method of feeding which gives them no more claim to be classed as parasitic animals than the whole group of spiders which feed in a similar manner on living flies.
The Origin of the Suctorial Apparatus of Petromyzon.
This powerful suctorial apparatus is innervated entirely by the trigeminal nerve, so that here in its muscular arrangements any original segmental arrangement of the muscles of mastication might be expected to be visible. It consists of a large rod or piston, to which are attached powerful longitudinal muscles; a large muscle, the basilar muscle, which assists the piston in producing a vacuum, and annular muscles around the circular lip.
Turn now to the full-grown larval form, Ammocœtes, an animal in the case ofPetromyzon Planerias large as the full-grown Petromyzon, and seek for this musculature. There is, apparently, no sign of it, no suctorial apparatus whatever, only, as already mentioned, an oral chamber bounded by the lower and upper lips and the remains of the septum between it and the respiratory chamber—the velar folds. Attached to its walls a number of tentacles are situated, which form a fringe around and within the mouth. Most extraordinary is the contrast here between the larval and the adult stages; in the former, no sign of the suctorial apparatus, but simply tentacles and velar folds; in the latter, no sign of tentacles or of velar folds, but a massive suctorial apparatus.
In order, then, to understand the origin of the muscles of mastication, it is necessary to study the changes which occur at transformation, and thus to find out how the suctorial apparatus of the adult arises. This most important investigation has been undertaken by Miss Alcock, and owing to the kindness of Mr. Millington, of Thetford, we have been able to obtain a better series in the transformation process than has ever been obtained before. Miss Alcock has not yet published her researches, but has allowed me to make use of some of her facts.
An enormous proliferation of muscular tissue takes place with great rapidity during this transformation, which causes the disappearance of the tentacles, and gives origin to the suctorial apparatus. The starting point of this proliferation can be traced back in all cases to little groups of embryonic tissue found below the epithelial lining of the oral chamber in Ammocœtes. Of these groups the most conspicuous one is situated at the base of the large median ventral tentacles. Others are situated at the base of the tentacular ridge. Further, although this extraordinary change takes place in theperipheral organ, no marked difference occurs in the arrangement of the nerves issuing from the trigeminal motor centre, no new nerves are formed to supply the new muscles, but every motor nerve-fibre and the motor cell from which it arises increases enormously in size, and these giant nerve-fibres thus formed split into innumerable filaments corresponding with the proliferation of the muscular elements.
The clue, then, to the origin of the suctorial apparatus and of the nature of the original organs supplied by the trigeminal is afforded in this case, as in all other similar inquiries, by the central nervous system and its outgoing nerves. Here is always the citadel, the fixed seat of government, here is 'headquarters,' from which the answers to all our inquiries must originate.
Fig. 114.—Distribution of Trigeminal Nerve in Ammocœtes.ps. br., pseudo-branchial groove;met., nerve to lower lip, or metastomal nerve;t., nerve to tongue;tent., nerve to tentacles. The mandibular and internal maxillary nerves are coloured red; the purely sensory nerves to the external surface are coloured black.
Fig. 114.—Distribution of Trigeminal Nerve in Ammocœtes.ps. br., pseudo-branchial groove;met., nerve to lower lip, or metastomal nerve;t., nerve to tongue;tent., nerve to tentacles. The mandibular and internal maxillary nerves are coloured red; the purely sensory nerves to the external surface are coloured black.
Fig. 114.—Distribution of Trigeminal Nerve in Ammocœtes.
ps. br., pseudo-branchial groove;met., nerve to lower lip, or metastomal nerve;t., nerve to tongue;tent., nerve to tentacles. The mandibular and internal maxillary nerves are coloured red; the purely sensory nerves to the external surface are coloured black.
The Trigeminal Nerve of Ammocœtes.
Striking is the answer. In Fig.114, Miss Alcock has drawn the distribution of the trigeminal nerve as traced by her through a series of sections. It arises, as is well known, from two separate ganglia, of which the foremost gives rise to a purely cutaneous nerve, the ophthalmic nerve, and the hindmost to three nerves, the most posterior of which is purely cutaneous and passes tailwards over the ventral branchial region, as shown in the figure; the other two nerves, bothof which contain motor fibres, are called by Hatschek the mandibular and maxillary nerves. Of these the mandibular or velar nerve (met.) is a large, conspicuous nerve, which arises so separately from the rest of the trigeminal as almost to deserve the title of a separate nerve. When it leaves the large posterior ganglion, it passes into the anterior part of the velum, runs along with the tubular muscles, which it supplies, to the ventral surface as far as the junction of the lower lip with the thyroid plate, and has not been followed further by Hatschek. Miss Alcock, however, by means of serial sections, has traced it further, and shown that at this point it turns abruptly headwards to terminate in the muscles of the lower lip. If, then, as suggested, the lower lip represents the metastoma—the last pair of prosomatic appendages—then this mandibular or velar nerve represents that segmental nerve.
The other nerve—the maxillary nerve of Hatschek—which constitutes the larger part of the trigeminal, passes forwards from the ganglion, and at a point somewhere about the anterior region of the eyeball, divides into two, an external (blackin Fig.114) and an internal (redin Fig.114) nerve. The external branch is apparently entirely sensory, and supplies the external surfaces of the upper and lower lips. The internal branch is mainly motor, and supplies the muscles of the upper lip; it contains also the nerves of the tentacles.
The nerve to the median ventral tentacle (t.) or tongue leaves the internal division of the maxillary immediately after its separation from the external; it runs ventralwards, and at the same time passes internally until it reaches a position between the muco-cartilage and the epithelium lining the cavity of the throat. It then turns, and passing posteriorly (towards the tail) to the point where the median ventral tentacle is attached to the lower lip, it supplies some very rudimentary-looking muscles which run from the tentacle to the adjoining surface, and no doubt serve to move the tentacle from side to side. A portion of the nerve still continues to run along the side of the median ventral ridge, as far back as the point where the muscles of the hyoid segment pass round to the ventral side between the velum and the thyroid; in fact, this small nerve passes along the whole length of the median ventral ridge.
This description shows that the trigeminal nerve divides itself into two groups: the one represented black in the figure, which is purely cutaneous and sensory, corresponding, in the main, accordingto my theory, to the epimeral nerves of Limulus; the other coloured red, which supplies muscles belonging to the visceral or splanchnic muscle-group, and contains also the nerves to the tentacles.
This latter group, which is formed by two distinct well-defined nerves, viz. the mandibular and the internal branch of the maxillary, corresponds, according to my theory, to the amalgamated nerves of the prosomatic appendages, and is clearly divisible into three distinct nerves—
1. The lower lip-nerve or the metastomal nerve (met.).
2. The tongue-nerve (t.).
3. The nerve (tent.) to the upper lip and tentacles.
Of these three pairs of nerves it is suggested that the first pair were derived from the nerves to the metastomal appendage. The second pair of nerves ought, on this theory, originally to have supplied the pair of appendages immediately in front of the metastoma—that is, the pair of ectognaths, and therefore the ventral pair of tentacles, known as the tongue, would represent the last remnant of these ectognaths. Similarly, the other tentacles would represent the endognaths, and therefore the third pair of nerves would represent the fused nerves to these concentrated endognaths, which, in the Eurypterids, stand aloof from the ectognaths.
Let us consider these three propositions separately. In the first place, have we any right to attribute segmental value to the mandibular nerve? What evidence is there of segments in this region in Ammocœtes?
The Segment of the Lower Lip, or Metastomal Segment.
We have seen that in the branchial or mesosomatic region the segments corresponding to the mesosomatic appendages were mapped out by means of their supporting or skeletal structures, their segmental muscles, and their nervous arrangements, as well as by the arrangement of the branchiæ. Similarly, the segments in front of the branchial region, corresponding to the prosomatic appendages, ought to be definable by the same means, although, owing to the absence of branchiæ and the greater concentration in this region, the separate segments would probably not be so conspicuous.
The last segment considered was the segment belonging to the VIIth nerve corresponding to the opercular appendages of theEurypterid. The segment immediately in front of this is the next for consideration, viz. that corresponding to the chilarial appendages or metastoma; and as the basal part of this pair of appendages was fused with the basal part of the operculum, the one cannot be discussed without the other; therefore, the segment to which the lower lip belongs must be considered in connection with and not apart from the thyro-hyoid segments already dealt with.
In Chapter V., p.188, I stated that the supporting bars of the foremost mesosomatic segments, the thyro-hyoid segments, differed from the cartilaginous bars of the branchial segments, in that they were composed of muco-cartilage. Also in addition to the muco-cartilaginous skeletal bars, a ventral plate of muco-cartilage exists in Ammocœtes which covers over the thyroid gland.
Similarly in the prosomatic segments the skeletal bars are composed of muco-cartilage and the ventral plate of muco-cartilage continues forward as the plate of the lower lip. It is of special interest, in connection with the segments indicated by such supporting structures, to find that this special tissue is entirely confined to the head-region, and disappears absolutely at transformation, thus indicating the ancestral nature of the segments marked out by its presence.
This muco-cartilaginous skeleton is the key to the whole position, and requires, therefore, to be understood. It is of great importance, not only because it demonstrates the position of the segments in Ammocœtes which characterized its invertebrate ancestor, but also because it possesses a structure remarkably similar to that found in the head-plates of the most ancient fishes. For the present I will confine myself to the consideration of this muco-cartilaginous skeleton as evidence of the relationship of Ammocœtes to the Eurypterids, and in the next chapter will show how absolutely the same skeleton corresponds to that of the Cephalaspidæ, so that Ammocœtes is really a slightly modified Cephalaspid, the larval form of which was Eurypterid in character.
Fig. 115.—Dorsal half of Head-region of Ammocœtes.Tr., trabeculæ;Pit., pituitary space;Inf., infundibulum;Ser., median serrated flange of velar folds.
Fig. 115.—Dorsal half of Head-region of Ammocœtes.Tr., trabeculæ;Pit., pituitary space;Inf., infundibulum;Ser., median serrated flange of velar folds.
Fig. 115.—Dorsal half of Head-region of Ammocœtes.
Tr., trabeculæ;Pit., pituitary space;Inf., infundibulum;Ser., median serrated flange of velar folds.
Fig. 116.—Horizontal Section through the Anterior Part of Ammocœtes, immediately Ventrally to the Auditory Capsule.sk1-sk5, skeletal bars;m1-m5, striated visceral muscles;mt1-mt4, tubular muscles;br1-br3, branchiæ;tr., trabeculæ;inf., infundibulum;ped., pedicle;V., trigeminal nerve. Muco-cartilage,red; soft cartilage, blue; hard cartilage, purple.
Fig. 116.—Horizontal Section through the Anterior Part of Ammocœtes, immediately Ventrally to the Auditory Capsule.sk1-sk5, skeletal bars;m1-m5, striated visceral muscles;mt1-mt4, tubular muscles;br1-br3, branchiæ;tr., trabeculæ;inf., infundibulum;ped., pedicle;V., trigeminal nerve. Muco-cartilage,red; soft cartilage, blue; hard cartilage, purple.
Fig. 116.—Horizontal Section through the Anterior Part of Ammocœtes, immediately Ventrally to the Auditory Capsule.
sk1-sk5, skeletal bars;m1-m5, striated visceral muscles;mt1-mt4, tubular muscles;br1-br3, branchiæ;tr., trabeculæ;inf., infundibulum;ped., pedicle;V., trigeminal nerve. Muco-cartilage,red; soft cartilage, blue; hard cartilage, purple.
Fig. 117.—Sagittal Lateral Section through the Anterior Part of Ammocœtes.Lettering and colouring same as in Fig.116.aud., auditory capsule;j.v., jugular vein.
Fig. 117.—Sagittal Lateral Section through the Anterior Part of Ammocœtes.Lettering and colouring same as in Fig.116.aud., auditory capsule;j.v., jugular vein.
Fig. 117.—Sagittal Lateral Section through the Anterior Part of Ammocœtes.
Lettering and colouring same as in Fig.116.aud., auditory capsule;j.v., jugular vein.
In Chapter IV., Figs. 63, 64, I have given a representation of the ventral and dorsal views of an Ammocœtes cut in half horizontally. Such a section shows with great clearness the series of branchial appendages with their segmental muscles and cartilaginous bars which form the branchial segments innervated by the IXth and Xth nerves, according to my view of the branchial unit. As is seen (Fig. 64 or 115), the skeletal bar of the hyoid or opercular appendage, which is clearly serially homologous with the other branchial bars, is composed of muco-cartilage, and not of cartilage. If we follow this series of horizontal sections nearer to the origin of the cartilaginous bars from the sub-chordal cartilaginous rod on each side of the notochord, we obtain a picture, as in Fig.116, in which each branchial segment is defined by the section of the branchial cartilaginous bar (sk4,sk5), by the section of the separate branchiæ (br2,br3), and by the separate segmental muscles arranged round each bar, these muscles being partly ordinary striated (m4,m5), partly tubular (mt3,mt4). The uppermost of these branchial segments shows the same arrangement; (sk3) is the branchial skeletal bar, which is now composed of muco-cartilage, not cartilage; (br1) is the branchiæ in the same situation as the others, but here composed of glandular rather than of respiratory epithelium, while the ordinary striated branchial muscles of this segment are marked as (m3), being separated from the tubular muscles of the segment (mt2), owing to the large size of the blood-space in which these latter muscles are lying. In front of this segment so defined we see again another well-marked skeletal bar (sk2) of muco-cartilage, evidently indicating a similar segment anterior to the hyoid segment. In connection with this bar there are no branchiæ, but again we see two sets of visceral muscles, the one ordinary striated, marked (m2), and the other tubular, marked (mt1). Here, then, the section indicates the existence of a segment of the same character as the posteriorly situated branchial segments but belonging to a non-branchial region—a segment which would represent a non-branchial appendage, the last, therefore, of the prosomatic appendages. Let us, then, followout these two segmental muco-cartilaginous bars and their attendant muscles, and see to what sort of segments their investigation leads.
The bar which comes first for consideration (sk3) arises immediately behind the auditory capsule from the first branchial cartilage very soon after it leaves the sub-chordal cartilaginous ligament; the soft cartilage of the sub-chordal ligament ceases abruptly in its extension along the notochord at the place where the hard cartilage of the parachordal joins it, and in a sense it may be said to leave the notochord at this place and pass into the basal part of the first branchial bar. The most anterior continuation of this branchial system is this muco-cartilaginous bar (sk3), which passes forward and ventralwards, being separated from the axial line by the auditory capsule (cf.Fig. 118, A, B, C). Its position is well seen in a sagittal section, such as Fig.117. It follows absolutely the line of the pseudo-branchial groove (ps. br., Fig.114), and ventrally joins the plate of muco-cartilage which covers the thyroid gland. It forms a thickened border to this plate anteriorly, just as the branchial cartilaginous bars border it posteriorly. In fact, it behaves with respect to the hyoid segment in a manner similar to the rest of the cartilaginous bars with respect to their respective segments.
It represents, although composed of muco-cartilage, the cartilaginous bar of the operculum in Limulus, which also forms the termination of the branchial cartilaginous system, as fully explained in Chapter III.; it may therefore be called the opercular bar.
The next bar (sk2) is extremely interesting, as we are now out of the branchial or mesosomatic region, and into the region corresponding to the prosoma. It starts from a cartilaginous projection made of hard cartilage, just in front of the auditory capsule, called by Parker the 'pedicle of the pterygoid'—a projection (ped.) which defines the posterior limit of the trabeculæ on each side, where they join on to the parachordals,—and winding round and below the auditory capsule, joins the opercular bar (cf.Fig.118), to pass thence into and form part of the muco-cartilaginous plate of the lower lip. In the section figured (Fig.116), this projection of hard cartilage is not directly continuous with (sk2), owing to a slight curvature in the bar; the next few sections show clearly the connection between (ped.) and (sk2), and consequently the complete separation by means of this bar of the hyoid segment from the segment in front.
Fig. 118.—Skeleton of Head-Region of Ammocœtes. A, Lateral View; B, Ventral View; C, Dorsal View.Muco-cartilage,red; soft cartilage,blue; hard cartilage,purple.sk1,sk2,sk3, skeletal bars;c.e., position of pineal eye;na. cart., nasal cartilage;ped., pedicle;cr., cranium;nc., notochord.
Fig. 118.—Skeleton of Head-Region of Ammocœtes. A, Lateral View; B, Ventral View; C, Dorsal View.Muco-cartilage,red; soft cartilage,blue; hard cartilage,purple.sk1,sk2,sk3, skeletal bars;c.e., position of pineal eye;na. cart., nasal cartilage;ped., pedicle;cr., cranium;nc., notochord.
Fig. 118.—Skeleton of Head-Region of Ammocœtes. A, Lateral View; B, Ventral View; C, Dorsal View.
Muco-cartilage,red; soft cartilage,blue; hard cartilage,purple.sk1,sk2,sk3, skeletal bars;c.e., position of pineal eye;na. cart., nasal cartilage;ped., pedicle;cr., cranium;nc., notochord.
In the figures, the hard cartilage is coloured purple, the soft cartilage blue, and the muco-cartilage red, so that the position of this bar is well shown. This bar may be looked upon as bearing the same relation to the muco-cartilaginous plate of the lower lip as the opercular bar does to the muco-cartilaginous plate over the thyroid; and seeing that these two plates form one continuous ventral head-shield of muco-cartilage (Fig.118, B), and also that this bar fuses with the opercular bar, we may conclude that the segment represented by the lower lip is closely connected with the hyoid or opercular segments. In other words, if the lower lip arose from the metastoma, then this pair of skeletal bars might be called the metastomal bars, which formed the supporting skeleton of the last pair of prosomatic appendages and, as is likely enough, arose in connection with the posterior lateral horns of the plastron; these posterior lateral horns, like the rest of the plastron, would give rise to hard cartilage, and so form in Ammocœtes the two lateral so-called pterygoid projections.
In the branchial region the muscles which marked out each branchial segment were of two kinds—ordinary striated visceral muscles and tubular muscles. Of these the former represented the dorso-ventral muscles of the branchial appendages, while the latter formed a separate group of dorso-ventral muscles with a separate innervation which may have been originally the segmental veno-pericardial muscles so characteristic of Limulus and the scorpions. In Figs. 116, 117, the grouping of these muscles in each branchial segment is well shown, and it is immediately seen that the hyoid segment possesses its group of striated visceral muscles (m3) supplied by the VIIth nerve in the same manner as the posterior groups, as has already been pointed out by Miss Alcock in her previous paper. Passing to the segment in front, Fig.116shows that the group of visceral muscles (m2) corresponds in relative position with respect to the metastomal bar to the hyoid muscles with respect to the opercular bar or to the branchial visceral muscles with respect to each branchial bar. What, then, is this muscular group? The series of sections show that these are the dorso-ventral muscles belonging to the lower lip, which, as seen in Fig.119(M.), form a well-marked muscular sheet, whose fibres interlace across the mid-ventral line of the lower lip. This group of lower lip-muscles is very suggestive, for these muscles arise, not from the trabeculæ, but from the front dorsal region of the cranium, just in front of the two lateraleyes. In Fig.117the dorsal part is seen cut across on its way to its dorsal attachment. Such an origin is reminiscent of the tergo-coxal group of muscles, arising, as they do, from the primordial cranium and the tergal carapace, and suggests at once that when the chilarial appendages expanded to form a metastoma, their tergo-coxal muscles formed a sheet of muscles similar to those of the lower lip of Ammocœtes, by which the movements of the metastoma were effected. The posterior limit of these muscles ventrally marks out the junction of the segment of the lower lip with that of the thyroid; in other words, indicates where the metastoma had fused ventrally with the operculum (Fig.117).
Fig. 119.—Ventral View Of Head-Region of Ammocœtes.Th., thyroid gland;M., lower lip, with its muscles.
Fig. 119.—Ventral View Of Head-Region of Ammocœtes.Th., thyroid gland;M., lower lip, with its muscles.
Fig. 119.—Ventral View Of Head-Region of Ammocœtes.
Th., thyroid gland;M., lower lip, with its muscles.
Besides the striated visceral muscles, each branchial segment possesses its own tubular muscles, shown in Fig.116(mt3) and (mt4). As the section shows, there is clearly a group of tubular muscle-fibres belonging to the hyoid segment (mt2), and also another group belonging to the segment in front of the hyoid (mt1); so that, judging from this section, each of these segments possesses its own tubular musculature just as do the branchial segments, the difference being that the tubular muscles are more separated from the striated visceral group than in the true branchial segments, owing to the size of the blood-spaces surrounding them. What, then, are these two groups of muscles? Tracing them in the series of sections, both groups are seen to belong to the system of velar muscles, forming an anterior and a posterior group respectively; and we see, further, that there is not the slightest trace of any tubular muscles anterior to these muscles of the velum.
In the living Ammocœtes the velar folds on each side can be seento move synchronously with the movements of respiration, contracting at each expiration, and thus closing the slit by which the oral and respiratory chambers communicate, and so forcing the waters of respiration through the gill-slits, as described by Schneider. Such a fact is clear evidence that these tubular muscles of the velar folds belong to the same series as the tubular muscles of the branchial segments, so that if, as I have already suggested, the latter muscles were originally the veno-pericardial muscles of segments corresponding to the branchial appendages, then the former would represent the veno-pericardial muscles of the segments corresponding to the opercular and metastomal appendages. What, then, are these velar folds, and how is it that the tubular muscles of these two segments become the velar muscles? I will consider, in the first instance, the posterior group of muscles (mt2) in Fig.116.
It has already been pointed out that the tubular muscles of the branchial segments are dorso-ventral, but do not run with the ordinary constrictors, having separate attachments and running part of their course internally to and part externally to the ordinary constrictors. At first sight, as is usually stated, the hyoid segment does not appear to possess tubular muscles at all. If, however, we follow the posterior group of velar muscles (mt2), we see (Fig.117) that they pass between the auditory capsule and the opercular bar (sk3) of muco-cartilage to reach the region of the jugular vein (j.v.) posteriorly to the auditory capsule, so that their dorsal origin bears the same relation to the hyoid segment as the dorsal attachment of the rest of the tubular muscles to their respective segments. Further, these muscles run along the length of the velar fold, and are attached ventrally on each side of the thyroid gland, so that their ventral attachment also corresponds in position, as regards the hyoid segment, with the ventral attachment of the rest of the tubular muscles as regards their respective segments.
This ventral attachment is shown in Fig.119on each side of the thyroid, and in Fig.120(mt2); while in Fig.117the fibres are seen converging to this ventral position. In other words, this large posterior muscle of the velar folds is a dorso-ventral muscle, and would actually take the same position in the hyoid segment as the dorso-ventral tubular muscles in the other branchial segments, if the velum were put back into its original position as the septum terminating the branchial chamber. Conversely, the presence of thesehyoid tubular muscles in the velum gives evidence that the opercular segment takes part in the formation of the septum, as already suggested.
Miss Alcock, in her paper, speaks of tubular muscles belonging to the hyoid segment, which are attached to the muco-cartilage. Schaffer also speaks of certain tubular muscles belonging to the velar group as piercing the muco-cartilage (h. r. s.) in his figures 24 and 25,i.e.the metastomal bar, near its junction with the opercular bar. In my specimens there is a distinct group of tubular muscles which pierce the opercular bar of muco-cartilage at its junction with the metastomal bar, and pass into the posterior group of velar muscles. They clearly belong to the hyoid segment, as Miss Alcock supposed, but are not attached to the muco-cartilage. It is possible that they represent a different group to those already considered, and suggest the possibility that this opercular or thyro-hyoid segment is double with respect to its original veno-pericardial muscles as well as in other respects.
The anterior group of tubular muscles (mt1, Figs. 116, 117) belonging to the same segment as the metastomal bar must now be taken into consideration. Very different is their origin to that of the posterior group: they arise close up against the eye, and have given rise to Kupffer's and Hatschek's misconception that the superior oblique muscle of the eye arises from a part of the velar musculature. Naturally, as Neal has pointed out, they have nothing to do with the eye-muscles; the superior oblique muscle is plainly in its true place entirely apart from these velar muscles, which form the foremost group of the segmental tubular muscles. They pass into the anterior part of the velar folds and run round to the ventral side just in the same way as does the posterior group. This anterior group of tubular muscles represents the veno-pericardial muscle of the segment immediately in front of the opercular,i.e.the metastomal segment, and is the foremost of these veno-pericardial muscles. Its presence shows that the velar folds, formed as they were by the breaking down of the septum, are in reality part of two segments, viz. the opercular and the metastomal, which have fused together in their basal parts, and by such fusion have caused the inter-relationship between the VIIth and Vth nerves, so apparent in the anatomy of the vertebrate cranial nerves.
A further piece of evidence that this anterior portion of the velumbelongs to the same segment as the lower lip is the fact that in addition to the tubular muscles a single ordinary striated muscle is found in the velum which, like the muscles of the lower lip, is innervated by this same mandibular nerve.
This muscle is attached laterally to the muco-cartilage of the metastomal bar (sk2) at its junction with the muco-cartilage of the lower lip, and spreads out into a number of strands which are attached at intervals along the whole length of the free anterior edge of the velum. It is the only non-tubular muscle belonging to the velum, and by its contraction it draws the anterior portions of the velar folds apart from each other, and so opens the slit between them, through which the food and mud must pass. Clearly from its position it does not belong to the original tergo-coxal group of muscles as do those of the lower lip; it must have been one of the intrinsic muscles of the metastoma itself.
This anterior portion of the velar folds affords yet another striking hint of the correctness of my comparison of the lower lip segment of Ammocœtes with the chilaria of Limulus or the metastoma of Eurypterus; for the most dorsal anterior portion, which at its attachment possesses a wedge of muco-cartilage, forms a separate, well-defined, rounded basal projection markedSer.in Fig.115, andBin the accompanying Fig.120. This is that part of the velar folds which comes together in the middle line and closes the entrance into the respiratory chamber. The epithelial surface here is most striking and suggestive, for it is markedly serrated, being covered with a large number of closely-set projections or serræ. The serration of the surface here is of so marked a character that Langerhans considered this part of the velar folds to act as a masticating organ, grinding and rasping the food and mud which passed through the narrow slit. In fact, Langerhans supposed that this portion of the velum acted in a manner closely resembling the action of the gnatho-bases of the prosomatic appendages in Limulus or the Eurypteridæ.
This suggestion of Langerhans is surely most significant, considering that this somewhat separate portion of the velum, to which he assigns such a function, is in the very place where the gnathite portion of the metastomal appendages would have been situated if it were true that the lower lip and anterior portion of the velum of Ammocœtes were derived from the metastoma.
In addition to this marked serrated edge the whole surface ofthe anterior portion of the velum is covered over with a scale-like or tubercular pattern remarkably like the surface-ornamentation seen in many of the members of the ancient group Eurypteridæ. In Fig.121I give a picture of this surface-marking of the velum. It is striking to see that just as in the case of the invertebrate this marking and these serræ are formed simply by the cuticular surface of the epithelial cells; a surface which, according to Wolff, possibly contains chitin. The interpretation which I would give of the velar folds is therefore as follows:—
They represent the fused basal parts of the opercular and metastomal appendages, the gnatho-bases of the latter still retaining in a reduced degree their rasping surfaces, because, owing to their position on each side of the opening into the respiratory chamber they were still able to manipulate the food as it passed by them after the closure of the old mouth.
Fig. 120.—Ammocœtes cut open in Mid-Ventral Line to show Position of Velum; Velar Folds removed on one side.tr., trabeculæ; vel., velum;B., anterior gnathic portion of velum;ps. br., pseudo-branchial groove;m2, muscles of lower lip segment;m3, muscles of thyro-hyoid segment;mt2, insertion of tubular muscles of velum near thyroid.
Fig. 120.—Ammocœtes cut open in Mid-Ventral Line to show Position of Velum; Velar Folds removed on one side.tr., trabeculæ; vel., velum;B., anterior gnathic portion of velum;ps. br., pseudo-branchial groove;m2, muscles of lower lip segment;m3, muscles of thyro-hyoid segment;mt2, insertion of tubular muscles of velum near thyroid.
Fig. 120.—Ammocœtes cut open in Mid-Ventral Line to show Position of Velum; Velar Folds removed on one side.
tr., trabeculæ; vel., velum;B., anterior gnathic portion of velum;ps. br., pseudo-branchial groove;m2, muscles of lower lip segment;m3, muscles of thyro-hyoid segment;mt2, insertion of tubular muscles of velum near thyroid.
Fig. 121.—Surface View of Anterior Surface of Velum.
Fig. 121.—Surface View of Anterior Surface of Velum.
Fig. 121.—Surface View of Anterior Surface of Velum.
The whole evidence points irresistibly to the conclusion that the mandibular or velar nerve of the trigeminal does supply a splanchnicsegment which is, in all respects, comparable with the segments supplied by the facial, glossopharyngeal, and vagus nerves, except that it does not possess branchiæ. This simply means that the appendages which these nerves originally supplied were prosomatic, not mesosomatic, and corresponded, therefore, to the chilarial or metastomal appendages.
A comparison of the ventral surface of Slimonia, as given in Fig.8, p.27, with that of Ammocœtes (Fig.119), when the thyroid gland and lower lip muscles have been exposed to view, enables the reader to recognize at a glance the correctness of this conclusion.
The Tentacular Segments and the Upper Lip.
Anterior to this metastomal segment, Fig.116shows a group of visceral muscles,m1, and yet again a muco-cartilaginous bar,sk1, but, as already stated, no tubular muscles. These visceral muscles indicate the presence in front of the lower lip-segment of one or more segments of the nature of appendages. The muscles in question (m1) are the muscles of the upper lip, the skeletal elements form a pair of large bars of muco-cartilage (sk1), which start from the termination of the trabeculæ, and pass ventralwards to fuse with the muco-cartilaginous plate of the lower lip (Figs. 117 and 118). This large bar forms the tentacular ridge on each side, and gives small projections of muco-cartilage into each tentacle. In addition to this tentacular bar, a special bar of muco-cartilage exists for the fused pair of median tentacles, the so-called tongue, which extends in the middle line along the whole length of the lower lip, being separated from the muco-cartilaginous plate of the lower lip by the muscles of the lower lip. This tongue bar of muco-cartilage joins with the muco-cartilage of the lower lip at its junction with the thyroid plate, and also with the tentacular bar just before the latter joins the muco-cartilaginous plate of the lower lip. This arrangement of the skeletal tissue suggests that the pair of tentacles known as the tongue stand in a category apart from the rest of the tentacles; a suggestion which is strongly confirmed by the separate character of its nerve-supply, as already mentioned.
For three reasons, viz. the separateness both of their nerve-supply and of their skeletal tissue, and the importance they assume at transformation, this pair of ventral tentacles must, it seems to me, be putinto a separate category from the rest of the tentacles. On the other hand, the innervation of the rest of the tentacles by a single nerve which sends off a branch as it passes each one, together with the concentration of their skeletal elements into a single bar, with projections into each tentacle, points directly to the conclusion that these tentacles must be considered as a group, and not singly.
I suggest that these tentacles are the remains of the ectognaths and endognaths; the tongue representing the two ectognaths, and the four tentacles on each side the four pairs of endognaths.
As we see, this method of interpretation attributes segmental value to the tentacles, a conclusion which is opposed to the general opinion of morphologists, who regard them as having no special morphological importance, and certainly no segmental value. On the other hand, the importance of the pair of ventral tentacles, the 'tongue' of Rathke, which lie in the mid-line of the lower lip, has been shown by Kaensche, Bujor, and others, all of whom are unanimous in asserting that at transformation they are converted into that large and important organ the piston or tongue of the adult Petromyzon. It is supposed that the rest of the tentacles vanish at transformation, being absorbed; they appear to me rather to take part in the formation of the sucking-disc, so that I am strongly inclined to believe that the whole of the remarkable suctorial apparatus of Petromyzon is derived from the tentacles of Ammocœtes. In other words, on my view, a conversion of the prosomatic appendages into a suctorial apparatus takes place at transformation, just as is frequently the case among the Arthropoda.
It is to the arrangement of the muscles that we look for evidence of segmental value. As long as it was possible to look upon these tentacles as mere sensory feelers round the mouth entrance, it was natural to deny segmental value to them. Matters are now, however, totally different since Miss Alcock's discovery of the rudimentary muscles at the base of the tentacles and their development at transformation. If these muscles represent some of the appendage muscles belonging to the foremost prosomatic segments just as the ocular muscles represent the dorso-ventral somatic muscles of those same segments, then we may expect ultimately to be able to give as good evidence of segmentation in their case as I have been able to give in the case of these latter muscles; for the two sets of muscles are curiously alike, seeing that the eye-muscles do not develop untiltransformation, but throughout the Ammocœtes stage remain in almost as rudimentary a condition as the tentacular muscles.
Another difficulty with respect to the tentacles is the determination of the number of them, owing to the fact that in addition to what may be called well-defined tentacles a large number of smaller tactile projections are found on the surface of the upper lip, as is seen in Fig.115. In the very young condition, 7 or 8 mm. in length, it is easier to make sure on this point. At this stage they may be spoken of as arranged in two groups: an anterior small group and a posterior larger group. The anterior group consists of a pair of very small tentacles and a very small median tentacle, all three situated quite dorsally in the front part of the upper lip. The posterior group, which is separate from the anterior, consists of five pairs of much larger tentacles, the most ventral pair in the mid-line ventrally on the lower lip being fused together to form the large ventral median tentacle or tongue already mentioned. This pair, according to Shipley, is markedly larger than the others. There are, therefore, five conspicuous tentacles on each side, and in front of them a smaller pair and a small median dorsal one. In the very young condition the accessory projections above-mentioned are not present, or at all events are not conspicuous, and the tentacles are also markedly larger in comparison to the size of the animal than in the older condition, where they have distinctly dwindled.
This posterior group of five conspicuous tentacles is the one which I suggest represents the four endognaths and one ectognath. What the significance of the small anterior group is, I know not. It is possible that the cheliceræ are represented here, for they are situated distinctly anterior to the other group; I know, however, of no sign of a markedly separate innervation to these most dorsal tentacles such as I should have expected to find if they represented the cheliceræ.
The muscles of the upper lip, which distinctly belong to the visceral and not to the somatic musculature, form part of the foremost segments, and in these muscles the tentacular nerve reaches its final destination. From their innervation, then, they must have belonged to the same appendages as the tentacles supplied by the tentacular nerve,i.e.to the endognaths. What conclusion can we form as to the probable origin of the upper lip of Ammocœtes? Since the oral chamber was formed by the forward growth of the metastoma,i.e.the lower lip of Ammocœtes, it follows that the upperlip is the continuation forwards of the original ventral surface of such an animal as Limulus or a member of the scorpion group, where there is no metastoma, and corresponds to the endostoma, as Holm calls it, of Eurypterus. This termination of the ventral surface in all these animals is made up of two parts: (1) Of sternites composing the true median ventral surface of the body, called by Lankester the pro- and meso-sternites; and (2) of the sterno-coxal processes of the foremost prosomatic appendages, called in the case of Limulus gnathites, because they are the main agents in triturating the food previously to its passage into the mouth. In Limulus, a conjoined pro-mesosternite forms the median ventral wall to which the sterno-coxal processes are attached on each side, and in Phrynus and Mygale a well-marked pro-sternite and meso-sternite are present, forming the posterior limit of the olfactory opening. In Buthus and the true scorpions the sterno-coxal processes of the 2nd, 3rd, and 4th prosomatic appendages take part in surrounding the olfactory tubular passage; in Thelyphonus only the processes of the 2nd pair of prosomatic appendages play such a part, the pro-sternite not being present (cf.Fig.97).
Seeing, then, what a large share the sterno-coxal processes of one or more of these prosomatic appendages plays in the formation of this endostoma, and seeing also that the nerve which supplies the upper lip-muscles in Ammocœtes is the same as that supplying the tentacles which are attached to the upper lip, it appears to me more probable than not that the muscles in question are the vestiges of the sterno-coxal muscles. These muscles differ markedly in their attachments from the muscles of the lower lip, for whereas the latter resemble the tergo-coxal group in their extreme dorsal attachment, the former resemble the sterno-coxal group in their attachment to what corresponds to the endostoma.
This interpretation of the meaning of the transformation process is in accordance with all the previous evidence both from the side of the palæostracan as from the side of the vertebrate, for it signifies that a dwindling process has taken place in the foremost of the original prosomatic appendages—the cheliceræ and the endognaths; while, on the contrary, the ectognath and the metastoma have continued to increase in importance right into the vertebrate stage. This process is simply a continuation of what was already going on in the invertebrate stage, for whereas in Eurypterus and other casesthe cheliceræ and endognaths had dwindled down to mere tentacles, the ectognath was the large swimming appendage, and the metastoma was on the upward grade from the two insignificant chilaria of Limulus.
The transformation of these foremost appendages into a suctorial apparatus is very common among the arthropods, as is seen in the transformation of the caterpillar into the butterfly, and it is in accordance with the evidence that the main mass of that suctorial apparatus should be formed from appendages corresponding to the ectognath and metastoma rather than from the four endognaths. In all probability thenucleus masticatoriusof the trigeminal nerve with its innervation of the great muscles of mastication is evidence of the continued development of the musculature of these two last prosomatic appendages, just as the descending root of the Vth demonstrates the further disappearance of all that belongs to the foremost prosomatic appendages. As yet, however, as far as I know, the musculature of the head-region of Petromyzon has not been brought into line with that of other vertebrates, and until that comparative study has been completed it is premature to discuss the exact position of the masticating muscles of the higher vertebrates.
The analysis of these tentacular segments belonging to the trigeminal nerve presents greater difficulties than that of any of the other cranial segments, owing to the deficiency of our knowledge of what occurs at transformation. Light is required not only on the origin of the new muscles but also on the origin of the new and elaborate cartilages which are newly formed at this time.
Miss Alcock has not yet worked out the origin of all these cartilages and muscles, so that we are not yet in a position to analyze the trigeminal supply in Petromyzon into its component appendage elements, an analysis which ought ultimately to enable us to determine from which appendage-muscles the masticating muscles in the higher vertebrates have arisen. As far as the muscles are concerned, she gives me the following information:—
The tongue-nerve supplies in Ammocœtes the rudimentary muscles which pass laterally from the base of the large ventral tentacle to the wall of the throat, and even in Ammocœtes must possess some power of moving that tentacle.
At transformation these muscles proliferate and develop enormously, and form the bulk of the large basilar muscle whichsurrounds the throat ventrally and laterally, and is the most bulky muscle in the suctorial apparatus.
The velar or mandibular nerve supplies in Ammocœtes the muscles of the lower lip. In Petromyzon it supplies also the longitudinal muscles of the tongue. The tongue-cartilage first develops in the region of the median ventral tentacle, and there the longitudinal tongue-muscles first begin to develop, not from the rudimentary muscles in the tongue but from those in the lower lip region.
In Ammocœtes the tentacular nerve supplies the rudimentary muscles in the tentacles and the muscles of the upper lip. The latter disappear entirely at transformation, and in Petromyzon the tentacular nerve supplies the circular, pharyngeal, and annular muscles, which are derived from the rudimentary tentacular muscles.
For the convenience of my reader I append here a table showing my conception of the manner in which the endognathal and ectognathal segments of the Palæostracan are represented in Ammocœtes. It shows well the uniform manner in which all the individual segmental factors have been fused together to represent the appearance of a single segment (van Wijhe's first segment) in the case of the four endognathal segments, but have retained their individuality in the case of the ectognathal segment.