Fig. 94.—Dorsal View of Brain and Camerostome of Galeodes.cam., camerostome;pr. ent., pre-oral entosclerite;l.l., dependent portion of camerostome;ph., pharynx;al., alimentary canal;n. op., median optic nerves;pl., plastron;v.c., ventral nerve chain; 2, 3, second and third appendages.
Fig. 94.—Dorsal View of Brain and Camerostome of Galeodes.cam., camerostome;pr. ent., pre-oral entosclerite;l.l., dependent portion of camerostome;ph., pharynx;al., alimentary canal;n. op., median optic nerves;pl., plastron;v.c., ventral nerve chain; 2, 3, second and third appendages.
Fig. 94.—Dorsal View of Brain and Camerostome of Galeodes.
cam., camerostome;pr. ent., pre-oral entosclerite;l.l., dependent portion of camerostome;ph., pharynx;al., alimentary canal;n. op., median optic nerves;pl., plastron;v.c., ventral nerve chain; 2, 3, second and third appendages.
At the base of the tubular passage formed by this modified first pair of antennæ the true mouth is found opening directly into the dilated pharynx, the muscles of which enable the act of suction to be carried out. The narrow œsophagus leads out from the pharynx and is completely surrounded by the supra- and infra-œsophageal nerve masses.
Huxley also describes the mouth of the scorpion in precisely the same position (cf. o, Fig.96).
In order to convey to my readers the antennæ-like character of the camerostome in Galeodes (Fig.101), and its position, I give a figure (Fig.94) of the organ from its dorsal aspect, after removal of the cheliceræ and their muscles. A side view of the same organ is given in Fig.95to show the feathered termination of the camerostome, and the position of the dependent accessory portion (l.l.) (Croneberg's 'untere Anhang') with its single long antenna-like feather. In both figures the alimentary canal (al.) is seen issuing from the conjoined supra- and infra-œsophageal mass.
As is seen in the figures, the bilateral character of the rostrum, as Croneberg calls it, is apparent not only in its feathered extremity but also in its chitinous covering, the softer median dorsal part (left white in figure) being bounded by two lateral plates of hard chitin, which meet in the middle line near the extremity of the organ. In all the members of the scorpion group, as is clearly shown in Croneberg's figures, the rostrum or camerostome is built up on the same plan as in Galeodes, though the antenna-like character may not be so evident.
Fig. 95.—Lateral View of Brain and Camerostome of Galeodes.gl. supr. œs., supra-œsophageal ganglion;gl. infr. œs., infra-œsophageal ganglion. The rest of the lettering same as in Fig.94.
Fig. 95.—Lateral View of Brain and Camerostome of Galeodes.gl. supr. œs., supra-œsophageal ganglion;gl. infr. œs., infra-œsophageal ganglion. The rest of the lettering same as in Fig.94.
Fig. 95.—Lateral View of Brain and Camerostome of Galeodes.
gl. supr. œs., supra-œsophageal ganglion;gl. infr. œs., infra-œsophageal ganglion. The rest of the lettering same as in Fig.94.
When we consider that the first pair of antennæ in the crustaceans are olfactory in function, Croneberg's observations amount to this—
In the arachnids and their allies the first pair of antennæ form a pre-oral passage or tube, olfactory in function; the small mouth, which opens directly into the pharynx, being situated at the end of this olfactory passage.
Croneberg's observations and conclusions are distinctly of very great importance in bringing the arachnids into line with the crustaceans, and it is therefore most surprising that they are absolutely ignored by Lankester and Miss Beck in their paper published in 1883, in which Latreille only is mentioned with respect to this organ, and his term "camerostome," or upper lip, is used throughout, in accordance with the terminology in Lankester's previous paper. That this organ is not only a movable lip or tongue, but essentially a sense-organ, almost certainly of smell and taste, as follows from Croneberg's conclusions, is shown by the series of sections which I have made through a number of young Thelyphonus (Fig.102).
Fig. 96.—Median Sagittal Section through a Young Thelyphonus.
Fig. 96.—Median Sagittal Section through a Young Thelyphonus.
Fig. 96.—Median Sagittal Section through a Young Thelyphonus.
I give in Fig.96a sagittal median section through the head-end of the animal, which shows clearly the nature of Croneberg's conception. At the front end of the body is seen the median eye (ce.),ois the mouth,Ph.the pharynx,œs.the narrow œsophagus, compressed between the supra-œsophageal (supr. œs.) and infra-œsophageal (infr. œs.) brain mass, which opens into the large alimentary canal (Al.);Olf. pass.is the olfactory passage to the mouth, lined with thick-set, very fine hairs, which spring from the hypostome (Hyp.) as well as from the large conspicuous camerostome (Cam.), which limits this tube anteriorly. The space between the camerostome and the median eye is filled up by the massive cheliceræ, which are not shown in this section, as they begin to appear in thesections on each side of the median one. The muscles of the pharynx and the muscles of the camerostome are attached to the pre-oral entosclerite (pr. ent.). The post-oral entosclerite is shown in section aspost. ent.The dorsal blood-vessel, or heart, is indicated atH.
In Fig.97I give a transverse section through another specimen of the same litter, to show the nature of this olfactory tube when cut across. Both sections show most clearly that we are dealing here with an elaborate sense-organ, the surface of which is partly covered with very fine long hairs, partly, as is seen in the figure, is composed of long, separate, closely-set sense-rods (bat.), well protected by the long hairs which project on every side in front of them, which recall to mind Bellonci's figure of the 'batonnets olfactives' on the antennæ of Sphæroma. Finally, we have the observation of Blanchard quoted by Huxley, to the effect that this camerostome is innervated by nerves from the supra-œsophageal ganglia which are clearly bilateral, seeing that they arise from the ganglion on each side and then unite to pass into the camerostome; in other words, paired olfactory nerves from the supra-œsophageal ganglia.
These facts demonstrate with wonderful clearness that in one group of the Arthropoda the olfactory antennæ have been so modified as to form an olfactory tube or passage, which leads directly into the mouth and so to the œsophagus of the animal, and, strikingly enough, this group, the Arachnida, is the very one to which the scorpions belong.
If for any cause the mouthoin Fig.96were to be closed, then the olfactory tube (olf. pass.) might still remain, owing to its importance as the organ of smell, and the olfactory tube would terminate blindly at the very spot where the corresponding tube does terminate in the vertebrate, according to the theory put forward in this book.
The Olfactory Tube of Ammocœtes.
In all cases where there is similarity of topographical position in the organs of the vertebrate and arthropod we may expect also to find similarity of structure. At first sight it would appear as though such similarity fails us here, for a cross-section of the olfactory tube in Petromyzon represents an elaborate organ such as is shown in Fig. 98, very different in appearance to the section across the olfactory passage of a young Thelyphonus given in Fig.97.
Fig. 97.—Transverse Section through the Olfactory Passage of a Young Thelyphonus.1 and 2, sections of first and second appendages.
Fig. 97.—Transverse Section through the Olfactory Passage of a Young Thelyphonus.1 and 2, sections of first and second appendages.
Fig. 97.—Transverse Section through the Olfactory Passage of a Young Thelyphonus.
1 and 2, sections of first and second appendages.
Fig. 98.—Transverse Section through the Olfactory Passage of Petromyzon.cart., nasal cartilage.
Fig. 98.—Transverse Section through the Olfactory Passage of Petromyzon.cart., nasal cartilage.
Fig. 98.—Transverse Section through the Olfactory Passage of Petromyzon.
cart., nasal cartilage.
As is seen, it is difficult to see any connection between these folds of olfactory epithelium and the simple tube of the scorpion. But in the nose, as in all other parts of the head-region of the lamprey, remarkable changes take place at transformation, and examination of the same tube in Ammocœtes demonstrates that the elaborate structure of the adult olfactory organ is actually derived from a much simpler form of organ, represented in Fig.99. Here, in Ammocœtes, the section is no longer strikingly different from that of the Thelyphonus organ, but, instead, most strikingly similar to it. Thus, again, it is shown that this larval form of the lamprey gives more valuable information as to vertebrate ancestry than all the rest of the vertebrates put together.
Fig. 99.—Transverse Section through the Olfactory Passage of Ammocœtes.cart., nasal cartilage.
Fig. 99.—Transverse Section through the Olfactory Passage of Ammocœtes.cart., nasal cartilage.
Fig. 99.—Transverse Section through the Olfactory Passage of Ammocœtes.
cart., nasal cartilage.
Still, even now the similarity between the two organs is not complete, for the tube in the lamprey opens on to the exterior on the dorsal surface of the head, while in the scorpion tribe it is situated ventrally, being the passage to the mouth and alimentary canal. In accordance with this there is no sign of any opening on the dorsal carapace of any of the extinct sea-scorpions or of the living land-scorpions, such as is so universally found in the cephalaspids, tremataspids, and lampreys. Here is a discrepancy of an apparently serious character, yet so wonderfully does the development of the individual recapitulate the development of the race, that this very discrepancy becomes converted into a triumphant vindication of thecorrectness of the theory advocated in this book, as soon as we turn our attention to the development of this nasal tube in the lamprey.
We must always remember not only the great importance of a larval stage for the unriddling of problems of ancestry, but also the great advantage of being able to follow more favourably any clues as to past history afforded by the development of the larva itself, owing to the greater slowness in the development of the larva than of the embryo. Such a clue is especially well marked in the course of development of Ammocœtes according to Kupffer's researches, for he finds that when the young Ammocœtes is from 5 to 7 mm. in length, some time after it has left the egg, when it is living a free larval life, a remarkable series of changes takes place with considerable rapidity, so that we may regard the transformation which takes place at this stage, as in some degree comparable with the great transformation which occurs when the Ammocœtes becomes a Petromyzon.
All the evidence emphasizes the fact that the latter transformation indicates the passage from a lower into a higher form of vertebrate, and is to be interpreted phylogenetically as an indication of the passage from the Cephalaspidian towards the Dipnoan style of fish. If, then, the former transformation is of the same character, it would indicate the passage from the Palæostracan to the Cephalaspid.
What is the nature of this transformation process as described by Kupffer?
It is characterized by two most important events. In the first place, up to this time the oral chamber has been cut off from the respiratory chamber by a septum—the velum—so that no food could pass from the mouth to the alimentary canal. At this stage the septum is broken through, the oral chamber communicates with the respiratory chamber, and the velar folds of the more adult Ammocœtes are left as the remains of the original septum. The other striking change is the growth of the upper lip, by which the orifice of the nasal tube is transferred from a ventral to a dorsal position. Fig.100, taken from Kupffer's paper, represents a sagittal section through an Ammocœtes 4 mm. long;l.l.is the lower lip,u.l.the upper lip, and, as is seen, the short oral chamber is closed by the septum,vel.Opening ventrally is a tube called the tube of the hypophysis,Hy., which extends close up to the termination of the infundibulum. On the anterior surface of this tube is the projection called by Kupffer the olfactory plakode. At this stage the upper lip grows with greatrapidity and thickens considerably, thus forcing the opening of the hypophysial tube more and more dorsalwards, until at last, in the full-grown Ammocœtes, it becomes the dorsal opening of the nasal tube, as already described. Here, then, in the hypophysial tube we have the original position of the olfactory tube of the vertebrate ancestor, and it is significant, as showing the importance of this organ, to find that such a hypophysial tube is characteristic of the embryological development of every vertebrate, whatever may be the ultimate form of the external nasal orifices.
The single median position of the olfactory organ in the Cyclostomata, in contradistinction to its paired character in the rest of the vertebrates, has always been a stumbling-block in the way of those who desired to consider the Cyclostomata as degenerated Selachians, for the origin of the olfactory protuberance, as a single median plakode, seemed to indicate that the nose arose as a single organ and not as a paired organ.
Fig. 100.—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. 100.—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. 100.—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.
On the other hand, the two olfactory nerves of Ammocœtes compare absolutely with the olfactory nerves of other vertebrates, and force one to the conclusion that this median organ of Ammocœtes arose from a pair of bilateral organs, which have fused in the middle line.
Fig. 101.—Galeodes.(From the Royal Natural History.)
Fig. 101.—Galeodes.(From the Royal Natural History.)
Fig. 101.—Galeodes.(From the Royal Natural History.)
The comparison of this olfactory organ with the camerostome gives a satisfactory reason for its appearance in the lowest vertebrates as an unpaired median organ; equally so, the history of the camerostome itself supplies the reason why the olfactory nerves are double, why the organ is in reality a paired organ and not a single median one. Thus, in a sense, the grouping of the fishes into Monorhinæ and Amphirhinæ has not much meaning, seeing that the olfactory organ is in all cases double.
Fig. 102.—Thelyphonus.(From the Royal Natural History.)
Fig. 102.—Thelyphonus.(From the Royal Natural History.)
Fig. 102.—Thelyphonus.(From the Royal Natural History.)
The evidence of the olfactory organs in the vertebrate not only confirms, in a most striking manner, the theory of the origin of thevertebrate from the Palæostracan, but points indubitably to an origin from a scorpion-like rather than a crustacean-like stock. To complete the evidence, it ought to be shown that the ancient sea-scorpions did possess an olfactory passage similar to the modern land-scorpions. The evidence on this question will come best in the next chapter, where I propose to deal with the prosomatic appendages of the Palæostracan group.
Summary.
The vertebrate olfactory apparatus commences as a single median tube which terminates dorsally in the lamprey, and is supplied by the two olfactory nerves which arise from the supra-infundibular portion of the brain. It is a long, tapering tube which passes ventrally and terminates blindly at the infundibulum in Ammocœtes. The dorsal position of the nasal opening is not the original one, but is brought about by the growth of the upper lip. The nasal tube originally opened ventrally, and was at that period of development known as the tube of the hypophysis.The evidence of Ammocœtes thus goes to show that the olfactory apparatus started as an olfactory tube on the ventral side of the animal, which led directly up to, and probably into, the œsophagus of the original alimentary canal of the palæostracan ancestor.Strikingly enough, although in the crustaceans the first pair of antennæ form the olfactory organs, no such free antennæ are found in the arachnids, but they have amalgamated to form a tube or olfactory passage, which leads directly into the mouth and œsophagus of the animal.This olfactory passage is very conspicuous in all members of the scorpion group, and, like the olfactory tube of the vertebrate, is innervated by a pair of nerves, which resemble those supplying the first pair of antennæ in crustaceans as to their origin from the supra-œsophageal ganglia.This nasal passage, or tube of the hypophysis, corresponds in structure and in position most closely with the olfactory tube of the scorpion group, the only difference being that in the latter case it opens directly into the œsophagus, while in the former, owing to the closure of the old mouth, it cannot open into the infundibulum.The evidence of the olfactory apparatus, combined with that of the optic apparatus, is most interesting, for, whereas the former points indubitably to an ancestor having scorpion-like affinities, the structure of the lateral eyes points distinctly to crustacean, as well as arachnid, affinities.Taking the two together the evidence is extraordinarily strong that the vertebrate arose from a member of the palæostracan group with marked scorpion-like affinities.
The vertebrate olfactory apparatus commences as a single median tube which terminates dorsally in the lamprey, and is supplied by the two olfactory nerves which arise from the supra-infundibular portion of the brain. It is a long, tapering tube which passes ventrally and terminates blindly at the infundibulum in Ammocœtes. The dorsal position of the nasal opening is not the original one, but is brought about by the growth of the upper lip. The nasal tube originally opened ventrally, and was at that period of development known as the tube of the hypophysis.
The evidence of Ammocœtes thus goes to show that the olfactory apparatus started as an olfactory tube on the ventral side of the animal, which led directly up to, and probably into, the œsophagus of the original alimentary canal of the palæostracan ancestor.
Strikingly enough, although in the crustaceans the first pair of antennæ form the olfactory organs, no such free antennæ are found in the arachnids, but they have amalgamated to form a tube or olfactory passage, which leads directly into the mouth and œsophagus of the animal.
This olfactory passage is very conspicuous in all members of the scorpion group, and, like the olfactory tube of the vertebrate, is innervated by a pair of nerves, which resemble those supplying the first pair of antennæ in crustaceans as to their origin from the supra-œsophageal ganglia.
This nasal passage, or tube of the hypophysis, corresponds in structure and in position most closely with the olfactory tube of the scorpion group, the only difference being that in the latter case it opens directly into the œsophagus, while in the former, owing to the closure of the old mouth, it cannot open into the infundibulum.
The evidence of the olfactory apparatus, combined with that of the optic apparatus, is most interesting, for, whereas the former points indubitably to an ancestor having scorpion-like affinities, the structure of the lateral eyes points distinctly to crustacean, as well as arachnid, affinities.
Taking the two together the evidence is extraordinarily strong that the vertebrate arose from a member of the palæostracan group with marked scorpion-like affinities.
CHAPTER VII
THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES
Comparison of the trigeminal with the prosomatic region.—The prosomatic appendages of the Gigantostraca.—Their number and nature.—Endognaths and ectognath.—The metastoma.—The coxal glands.—Prosomatic region of Eurypterus compared with that of Ammocœtes.—Prosomatic segmentation shown by muscular markings on carapace.—Evidence of cœlomic cavities in Limulus.—Summary.
Comparison of the trigeminal with the prosomatic region.—The prosomatic appendages of the Gigantostraca.—Their number and nature.—Endognaths and ectognath.—The metastoma.—The coxal glands.—Prosomatic region of Eurypterus compared with that of Ammocœtes.—Prosomatic segmentation shown by muscular markings on carapace.—Evidence of cœlomic cavities in Limulus.—Summary.
The derivation of the olfactory organs of the vertebrate from the olfactory antennæ of the arthropod in the last chapter is confirmatory proof of the soundness of the proposition put forward in Chapter IV., that the segmentation in the cranial region of the vertebrate was derived from that of the prosomatic and mesosomatic regions of the palæostracan ancestor. Such a segmentation implies a definite series of body-segments, corresponding to the mesomeric segmentation of the vertebrate, and a definite series of appendages corresponding to the splanchnic segmentation of the vertebrate.
Of the foremost segments belonging to the supra-œsophageal region characterized by the presence of the median eyes, of the lateral eyes, and of the olfactory organs, a wonderfully exact replica has been shown to exist in the pineal eyes, the lateral eyes, and the olfactory organ of the vertebrate, belonging, as they all do, to the supra-infundibular region.
Of the infra-œsophageal segments belonging to the prosoma and mesosoma respectively, the correspondence between the mesosomatic segments carrying the branchial appendages and the uterus, with those in the vertebrate carrying the branchiæ and the thyroid gland respectively, has been fully proved in previous chapters.
There remain, then, only the segments of the prosomatic region to be considered, a region which, both in the vertebrate and invertebrate, is never respiratory in function but always masticatory, suchmastication being performed in Limulus and its allies by the muscles which move the foot-jaws or gnathites, which are portions of the prosomatic appendages specially modified for that purpose, and in the vertebrates by the masticatory muscles, which are always innervated by the trigeminal or Vth cranial nerve. This comparison implies that the motor part of the trigeminal nerve originally supplied the prosomatic appendages.
The investigations of van Wijhe and of all observers since the publication of his paper prove that in this trigeminal region, as in the vagus region, a double segmentation exists, of which the ventral or splanchnic segments, corresponding to the appendages in the invertebrate, are supplied by the trigeminal nerves, while the dorsal or somatic segments, corresponding to the somatic segments in the invertebrate, are supplied by the IIIrd or oculomotor and the IVth or trochlear nerves—nerves which supply muscles moving the lateral eyes.
In accordance, then, with the evidence afforded by the nerves of the branchial segments, it follows that the muscles supplied by the motor part of the trigeminal ought originally to have moved the appendages belonging to a series of prosomatic segments. On the other hand, the eye-muscles ought to have belonged to the body-part of the prosomatic segments, and must therefore have been grouped originally in a segmental series corresponding to the prosomatic appendages.
The evidence for and against this conclusion will be the subject of consideration in this and the succeeding chapters. At the outset it is evident that any such comparison necessitates an accurate knowledge of the number of the prosomatic segments in the Gigantostraca and of the nature of the corresponding appendages.
In all this group of animals, the evidence as to the number of segments in either the prosomatic or mesosomatic regions is given by—
1. The number of appendages.
2. The segmental arrangement of the muscles of the prosoma or mesosoma respectively.
3. The segmental arrangement of the cœlomic or head-cavities.
4. The divisions of the central nervous system, or neuromeres, together with their outgoing segmental nerves.
It follows, therefore, that if from any cause the appendages are not apparent, as is the case in many fossil remains, or have dwindledaway and become insignificant, we still have the muscular, cœlomic, and nervous arrangements left to us as evidence of segmentation in these animals, just as in vertebrates.
In this prosomatic region, we find in Limulus the same tripartite division of the nerves as in the mesosomatic region, so that the nerves to each segment may be classed as (1) appendage-nerve; (2) sensory or dorsal somatic nerve, supplying the prosomatic carapace; (3) motor or ventral somatic nerve, supplying the muscles of the prosoma, and containing possibly some sensory fibres. The main difference between these two regions in Limulus consists in the closer aggregation of the prosomatic nerves, corresponding to the concentration of the separate ganglia of origin in the prosomatic region of the brain.
The number of prosomatic segments in Limulus is not evident by examination of the prosomatic carapace, so that the most reliable guide to the segmentation of this region is given by the appendages, of which one pair corresponds to each prosomatic segment.
The number of such segments, according to present opinion, is seven, viz.:—
(1) The foremost segment, which bears the cheliceræ.
(2, 3, 4, 5, 6) The next five segments, which carry the paired locomotor appendages; and
(7) The last segment, to which belongs a small abortive pair of appendages, known by the name of the chilaria, situated between the last pair of locomotor appendages and the operculum or first pair of mesosomatic appendages. These appendages are numbered from 1-7 in the accompanying drawing (Fig.103).
Of these seven pairs of appendages, the significance of the first and the last has been matter of dispute. With respect to the first pair, or the cheliceræ, the question has arisen whether their nerves belong to the infra-œsophageal group, or are in reality supra-œsophageal.
It is instructive to observe the nature and the anterior position of this pair of appendages in the allied sea-scorpions, especially in Pterygotus, where the only chelate organs are found in these long, antennæ-like cheliceræ. In Slimonia and in Stylonurus they are supposed by Woodward to be represented by the small non-chelate antennæ seen in Fig.8, B and C (p.27), taken from Woodward. If such is the case, then these figures show that a pair of appendages is missing in eachof these forms, for they possess only five free prosomatic appendages instead of six, as in Limulus and in Pterygotus. Similarly, Woodward only allowed five appendages for Pterygotus, so that his restorations were throughout consistent. Schmidt, inPterygotus osiliensishas shown that the true number was six, not five, as seen in his restoration given in Fig.8, A (p.27).
Fig. 103.—Ventral Surface of Limulus.(Taken fromKishinouye.)The gnathic bases of the appendages have been separated from those of the other side to show the promesosternite or endostoma (End.).
Fig. 103.—Ventral Surface of Limulus.(Taken fromKishinouye.)The gnathic bases of the appendages have been separated from those of the other side to show the promesosternite or endostoma (End.).
Fig. 103.—Ventral Surface of Limulus.(Taken fromKishinouye.)
The gnathic bases of the appendages have been separated from those of the other side to show the promesosternite or endostoma (End.).
With respect to Eurypterus, Schmidt figures an exceedingly minute pair of antennæ between the coxal joints of the first pair of appendages, thus making six pairs of appendages. Gerhard Holm, however, in his recent beautiful preparations from Schmidt's specimens and others collected at Rootziküll, has proved most conclusively that the cheliceræ of Eurypterus were of the same kind as those of Limulus. I reproduce his figure (Fig.104) showing the small chelate cheliceræ (1) overhanging the mouth orifice, just as in Limulus or in Scorpio.
So, also, since Woodward's monograph, Laurie has discovered inSlimonia acuminataa small median pair of chelate appendages exactly corresponding to the cheliceræ of Limulus, or of Eurypterus, or of Scorpio. We may, therefore, take it for granted that such was also the case in Stylonurus, and that the foremost pair of prosomatic appendages in all these extinct sea-scorpions were in the same position and of the same character as the cheliceræ of the scorpions.
Fig. 104.—Eurypterus Fischeri.(FromHolm.)
Fig. 104.—Eurypterus Fischeri.(FromHolm.)
Fig. 104.—Eurypterus Fischeri.(FromHolm.)
In the living scorpion and in Limulus the nerves to this pair of appendages undoubtedly arise from the foremost prosomatic ganglia, and the reason why they appear to belong to the supra-œsophageal brain-mass has been made clear by Brauer's investigations on the embryology of Scorpio; for he has shown that the cheliceral ganglia shift from the ventral to the dorsal side of the œsophagus during development, thus becoming pseudo-supra-œsophageal, though in reality belonging to the infra-œsophageal ganglia. This cheliceral pair of appendages is, in all probability, homologous with the second pair of antennæ in the crustacea.
I conclude, then, that the cheliceræ must truly be included in the prosomatic group, but that they stand in a somewhat different category to the rest of the prosomatic appendages, inasmuch as they take up a very median anterior and somewhat dorsal position, and their ganglia of origin are also exceptional in position.
Next for consideration come the chilaria (7 in Fig.103), which Lankester did not consider to belong to appendages at all, but to be a peculiar pair of sternites. Yet their very appearance, with their spinous hairs corresponding to those of the other gnathites and their separate nerve-supply, all point distinctly to their being a modified pair of appendages, and, indeed, the matter has been placed beyond doubt by the observations of Kishinouye, who has found embryologically that they arise in the same way as the rest of the prosomatic appendages, and belong to a distinct prosomatic segment, viz. the seventh segment. In accordance with this, Brauer has found that in the scorpion there is in the embryo a segment, whose appendages degenerate, which is situated between the segment bearing the last pair of thoracic appendages and the genital operculum—a segment, therefore, comparable in position to the chilarial segment of Limulus.
Coming now to the five locomotor appendages, we find that they resemble each other to a considerable extent in most cases, with, however, certain striking differences. Thus in Limulus they are chelate, with their basal joints formed as gnathites, except in the case of the fifth appendage, in which the extremity is modified for the purpose of digging in the sand. In Pterygotus, Slimonia, Eurypterus, the first four of these appendages are very similar, and are called by Huxley and Woodward endognaths; in all cases they possess a basal part or sterno-coxal process, which acts as a gnathite or foot-jaw, and a non-chelate tactile part, which possesses no prehensile power, and in most cases could have had no appreciable share in locomotion, called by Huxley and Woodward the palpus. These small palps were probably retractile, and capable of being withdrawn entirely under the hood. The fifth appendage is usually different, being a large swimming organ in Pterygotus, Eurypterus, and Slimonia (Figs. 8 and 104), and is known as the ectognath.
Finally, inDrepanopterus Bembycoides, as stated by Laurie, all five locomotor appendages are built up after the same fashion, the last one not being formed as a paddle-shaped organ or elongated asin Stylonurus, but all five possess no special locomotor or prehensile power. According to Laurie this is a specially primitive form of the group.
It is significant to notice from this sketch that with the absence of special prehensile terminations such as chelæ, or the absence of special locomotor functions such as walking or swimming, these appendages tend to dwindle and become insignificant, taking up the position of mere feelers round the mouth, and at the same time are concentrated and pressed closely together, so that their appendage-nerves must also be close together.
This sketch therefore shows us that—
Of the six foremost prosomatic appendages, the cheliceræ and the four endognaths were, at the time when the vertebrates first appeared, in very many cases dwindling away; the latter especially no longer functioned as locomotor appendages, but were becoming more and more mere palps or tentacles situated round the mouth, which could by no possibility afford any help to locomotion.
On the contrary, the sixth pair of appendages—the ectognaths—remained powerful, being modified in many cases into large oar-like limbs by which the animal propelled itself through the water.
It is a striking coincidence that those ancient fishes,Pterichthysand Bothriolepis, should have possessed a pair of large oar-like appendages.
At this time, then, in strong contrast to the endognaths, the ectognaths, or sixth pair of appendages, remained strong and vigorous. What about the seventh pair, the chilaria of Limulus?
Of all the prosomatic appendages these are the most interesting from the point of view of my theory, for whereas in the scorpion of the present day they have dwindled away and left no trace except in the embryo, in the sea-scorpions of old, far from dwindling, they had developed and become a much more important organ than the chilaria of Limulus.
In all these animals a peculiarly striking and unique structure is found in this region known by the name of the metastoma, or lip-plate (Figs. 8 and 104 (7)); it is universally considered to be formed by the fusion of the two chilarial appendages.
All observers are agreed that this lip-plate was freely movable. Nieskowski considers that the movement of the metastoma was entirely in a vertical direction, whereby the cleft which is seenbetween the basal joints of all the pairs of locomotor appendages could be closed from behind. Woodward says it no doubt represents the labium, and served more effectually to enclose the posterior part of the buccal orifice, being found exteriorly to the toothed edges of the ectognaths or maxillipedes. Schmidt agrees with Nieskowski, and looks on the mestasoma as forming a lower lip within which the bases of the ectognaths worked.
Fig. 105.—Diagram of Sagittal Median Section through A, Limulus, B, Eurypterus.
Fig. 105.—Diagram of Sagittal Median Section through A, Limulus, B, Eurypterus.
Fig. 105.—Diagram of Sagittal Median Section through A, Limulus, B, Eurypterus.
Quite recently Gerhard Holm has worked over again the very numerous specimens ofEurypterus Fischeri, which are obtainable at Rootziküll, and has thrown new light on the relation of the metastoma to the mouth-parts. His preparations show clearly that the true lower lip of Eurypterus was not the metastoma, for when the metastoma is removed another plate (End., Fig.105, B) situatedinternally to it is disclosed, which, in his view, corresponds to the sternite between the bases of the pro-somatic appendages in Limulus,i.e.to the sternite called by Lankester, the pro-mesosternite (End., Fig.103). This inner plate formed with the metastoma ((7) Fig. 105) and the ectognaths (6) a chamber closed posteriorly, within which the bases of the ectognaths worked. In other words, the removal of the metastoma discloses in Eurypterus the true anterior ventral surface of the animal which corresponds to that of Limulus, or of the scorpion group, with its pro-mesosternite and laterally attached gnathites or sterno-coxal processes. To this inner plate or pro-mesosternite Holm gives the name ofendostoma.
To the anterior edge of the endostoma a thinner membrane is attached which passes inwards in the direction of the throat, and forms, therefore, the lower lip (Hyp., Fig.105, B) of the passage of the mouth (olf. p.). This membrane bears upon its surface a tuft of hairs, which he thought were probably olfactory in function. Consequently, in his preliminary communication, he describes this lower lip as forming, in all probability, an olfactory organ; in his full communication he repudiates this suggestion, because he thinks it unlikely that such an organ would be situated within the mouth. I feel sure that if Holm had referred to Croneberg's paper, and seen how the true mouth in all the scorpion group is situated at the base of an olfactory passage, he would have recognized that his first suggestion is in striking accordance with the nature of the entrance to the mouth in other scorpions.
That Eurypterus also possessed a camerostome (cam.) seems to follow of necessity from its evident affinities both with Limulus and the scorpions. We see, in fact, that the mouth of these old sea-scorpions was formed after the fashion of Limulus, surrounded by masticatory organs in the shape of foot-jaws, and yet foreshadowed that of the scorpion, so that an ideal sagittal section of one of these old palæostracan forms would be obtained by the combination of actual sagittal sections through Limulus and a member of the scorpion group, with, at the same time, a due recognition of Holm's researches. Such a section is represented in Fig.105, B, in which I have drawn the central nervous system and its nerves, the median eyes (C.E.), the olfactory organs (Cam.), the pharynx (Ph.), œsophagus (œs.), and alimentary canal (Al.), but have not tried to indicate the lateral eyes. I have represented the prosomatic appendages by numbers (1-7), andthe foremost mesosomatic segments by numbers (8-13). I have placed the four endognaths and the nerves going to them close together, and made them small, mere tentacles, in recognition of the character of these appendages in Eurypterus, and have indicated the position and size of the large ectognath, with its separate nerve, by (6). If among the ancient Eurypterus-like forms, which were living at the time when vertebrates first appeared, there were some in which the ectognaths also had dwindled to a pair of tentacles, then such animals would possess a prosomatic chamber formed by a metastoma or accessory lip, within which were situated five pairs of short tactile appendages or tentacles. If the vertebrate were derived from such an animal, then the trigeminal nerve, as the representative of these prosomatic appendage-nerves, ought to be found to supply the muscles of this accessory lip and of these five pairs of tentacles in the lowest vertebrate.
This prosomatic or oral chamber, as it might be called, was limited posteriorly by the fused metastoma (7) and operculum (8), so that if in the same imaginary animal one imagines that the gill-chambers, instead of being separate, are united to form one large respiratory chamber, then, in such an animal, a prosomatic oral chamber, in which the prosomatic appendages worked, would be separated from a mesosomatic respiratory chamber by a septum composed of the conjoined basal portions of the mesosomatic operculum and the prosomatic metastoma, as indicated in the diagram. In this septum the nerves to the last prosomatic appendage (equivalent to the last part of the trigeminal in the vertebrate) and to the first mesosomatic (equivalent to the thyroid part of the facial) would run, as shown in the figure, close together in the first part of their course, and would separate when the ventral surface was reached, to pass headwards and tailwards respectively.
The Coxal Glands.
One more characteristic of these appendages requires mention, and that is the excretory glands situated at the base of the four endognaths known as the coxal glands. These glands are the main excretory organs in Limulus and the scorpions, and extend into the basal segments or coxæ of the four endognaths, not into those of the ectognaths or the chilaria (or metastoma). Hence their name, coxalglands; and, seeing the importance of the excretory function, it is likely enough that they would remain, even when the appendages themselves had dwindled away. With the concentration and dwindling of the endognaths these coxal glands would also be concentrated, so that in the diagram (Fig.105) they would rightly be grouped together in the position indicated (cox. gl.).
Such a diagram indicates the position of all the important organs of the head-region except the special organs for taste and hearing. These, for the sake of convenience, I propose to take separately, in order at this stage of my argument not to overburden the simplicity of the comparison I desire to make with too much unavoidable detail.
The Prosomatic Region of Ammocœtes.
Let us now compare this diagram with that of the corresponding region in Ammocœtes and see whether or no any points of similarity exist.
With respect to this region, as in so many other instances already mentioned, Ammocœtes occupies an almost unique position among vertebrates, for the region supplied by the trigeminal nerve—the prosomatic region—consists of a large oral chamber which was separated from the respiratory chamber in the very young stage by a septum which is subsequently broken through, and so the two chambers communicate.
This chamber is bounded by the lower lip ventrally, the upper lip and trabecular region dorsally, and the remains of the septum or velum laterally and posteriorly. It contains a number of tentacles arranged in pairs within the chamber so as to form a sieve-like fringe inside the circular mouth; of these, the ventral pair are large, fused together, and attached to the lower lip.
All the muscles belonging to this oral chamber are of the visceral type, and are innervated by the trigeminal nerve. In accordance with the evidence obtained up to this point this means that such an oral chamber was formed by the prosomatic appendages of the invertebrate ancestor, similarly to the oral chamber just figured for Eurypterus.
This chamber in the full-grown Ammocœtes is not only open to the respiratory chamber, but is bounded by the large upper lip (U.L., Fig.106, D). On the dorsal surface of this region, in front of thepineal eye (C.E.), is the most conspicuous opening of the olfactory tube (Na.), which olfactory tube passes from the dorsal region to the ventral side to terminate blindly at the very spot where the infundibulum comes to the surface of the brain. Here, also, is situated that extraordinary glandular organ known as the pituitary body (Pit.). A sagittal section, then, in diagram form, of the position of parts in the full-grown Ammocœtes, would be represented as in Fig.106, D.
But, as argued out in the last chapter, the diagram of the adult Ammocœtes must be compared with that of a cephalaspidian fish; the diagram of the palæostracan must be compared with the larval condition of Ammocœtes. In other words, Fig.106, B, must be compared with Fig.106, C, which represents a section through the larval Ammocœtes as it would appear if it reached the adult condition without any forward growth of the upper lip or any breaking through of the septum between the oral and respiratory chambers. The striking similarity between this diagram and that of Eurypterus becomes immediately manifest even to the smallest details. The only difference between the two, except, of course, the notochord, consists in the closure of the mouth opening (o), in Fig.106, B, by which the olfactory passage (olf. p.) of the scorpion becomes converted into the hypophysial tube (Hy.), Fig.106, C, and later into the nasal tube (Na.), Fig.106, D, of the full-grown Ammocœtes. That single closure of the old mouth is absolutely all that is required to convert the Eurypterus diagram into the Ammocœtes diagram.
Such a comparison immediately explains in the simplest manner a number of anatomical peculiarities which have hitherto been among the great mysteries of the vertebrate organization. For not only do the median eyes (C.E.) correspond in position in the two diagrams, and the infundibular tube (Inf.) and the ventricles of the brain (C.C.) correspond to the œsophagus (œs.) and the cephalic stomach (Al.), as already fully discussed; but even in the very place where the narrow œsophagus opened into the wider chamber of the pharynx (Ph.), there, in all the lower vertebrates, the narrow infundibular tube opens into the wider chamber of the membranoussaccus vasculosus(sac. vasc.). This is the last portion of the membranous part of the tube of the central nervous system which has not received explanation in the previous chapters, and now it is seen how simple its explanation is, how natural its presence—it represents the old pharyngeal chamber of the palæostracan ancestor.