Illustration: Figure 221Fig. 221. Two stages in the development of Pilidium.(After Metschnikoff.)ae.archenteron;oe.œsophagus;st.stomach;am.amnion;pr.d.prostomial disc;po.d.metastomial disc;c.s.cephalic sack (lateral pit).
Fig. 221. Two stages in the development of Pilidium.(After Metschnikoff.)ae.archenteron;oe.œsophagus;st.stomach;am.amnion;pr.d.prostomial disc;po.d.metastomial disc;c.s.cephalic sack (lateral pit).
2. The Echinoderm Group.—This group (figs.223,224and231C) is characterised by the presence of a longitudinalpostoralband of cilia, by the absence of special sense organs in the præoral region, and by the development of the body cavity as an outgrowth of the alimentary tract. The three typical divisions of the alimentary tract are present, and there is a more or less developed præoral lobe. This group only includes the larvæ of the Echinodermata.
3. The Trochosphere Group.—This group (figs.225,226) is characterised by the presence of a præoral ring of long cilia, the region in front of which forms a great part of the præoral lobe. The mouth opens immediately behind the præoral ring of cilia, and there is very often a second ring of short cilia parallel to the main ring, immediately behind the mouth. The function of the ring of short cilia is nutritive, in that its cilia are employed in bringing food to the mouth; while the function of the main ring is locomotive. A perianal patch or ring of cilia is often present (fig. 225A), and in many forms intermediate rings are developed between the præoral and perianal rings.
Illustration: Figure 222Fig. 222. A. Larva of Eurylepta auriculata immediately after hatching. Viewed from the side.(After Hallez.)m.mouth.B. Müller’s Turbellarian larva (probably Thysanozoon). Viewed from the ventral surface.(After Müller.) The ciliated band is represented by the black line.m.mouth;u.l.upper lip.
Fig. 222. A. Larva of Eurylepta auriculata immediately after hatching. Viewed from the side.(After Hallez.)m.mouth.B. Müller’s Turbellarian larva (probably Thysanozoon). Viewed from the ventral surface.(After Müller.) The ciliated band is represented by the black line.m.mouth;u.l.upper lip.
The præoral lobe is usually the seat of a special thickening of epiblast, which gives rise to the supraœsophageal ganglion of the adult. On this lobe optic organs are very often developed in connection with the supraœsophageal ganglion, and a contractile band frequently passes from this region to the œsophagus.
The alimentary tract is formed of the three typical divisions.
The body cavity is not developed directly as an outgrowth of the alimentary tract, though the process by which it originates is very probably secondarily modified from a pair of alimentary outgrowths.
Paired excretory organs, opening to the exterior and into the body cavity, are often present (fig. 226nph).
This type of larva is found in the Rotifera (fig. 217) (in which it is preserved in the adult state), the Chætopoda (figs.225and226), the Mollusca (fig. 218), the Gephyrea nuda (fig. 227), and the Polyzoa (fig. 228)[140].
Illustration: Figure 223Fig. 223. A. The larva of a Holothuroid.B. The larva of an Asteroid.m.mouth;st.stomach;a.anus;l.c.primitive longitudinal ciliated band;pr.c.præoral ciliated band.
Fig. 223. A. The larva of a Holothuroid.B. The larva of an Asteroid.m.mouth;st.stomach;a.anus;l.c.primitive longitudinal ciliated band;pr.c.præoral ciliated band.
4. Tornaria.—This larva (fig. 229) is intermediate in most of its characters between the larvæ of the Echinodermata (more especially the Bipinnaria) and the Trochosphere. It resembles Echinoderm larvæ in the possession of a longitudinal ciliated band (divided into a præoral and a postoral ring), and in the derivation of the body cavity and water-vascular vesicle from alimentary diverticula; and it resembles the Trochosphere in the presence of sense organs on the præoral lobe, in the existence of a perianal ring of cilia, and in the possession of a contractile band passing from the præoral lobe to the œsophagus.
Illustration: Figure 224Fig. 224. A larva of Strongylocentrus.(From Agassiz.)m.mouth;a.anus;o.œsophagus;d.stomach;c.intestine;v´.andv.ciliated ridges;w.water-vascular tube;r.calcareous rods.
Fig. 224. A larva of Strongylocentrus.(From Agassiz.)m.mouth;a.anus;o.œsophagus;d.stomach;c.intestine;v´.andv.ciliated ridges;w.water-vascular tube;r.calcareous rods.
5. Actinotrocha.—The remarkable larva of Phoronis (fig. 230), known as Actinotrocha, is characterised by the presence of (1) a postoral and somewhat longitudinal ciliated ring produced into tentacles, and (2) a perianal ring. It is provided with a præoral lobe, and a terminal or somewhat dorsal anus.
6. The larva of the Brachiopoda articulata(fig. 220).
The relationships of the six types of larval forms thus briefly characterised have been the subject of a considerable amount of controversy, and the following suggestions on their affinities must be viewed as somewhat speculative. The Pilidium type of larva is in some important respects less highly differentiated than the larvæ of the five other groups. It is, in the first place, without an anus; and there are no grounds for supposing that the anus has become lost by retrogressive changes. If for the moment it is granted that the Pilidium larva represents more nearly than the larvæ of the other groups the ancestral type of larva, what characters are we led to assign to the ancestral form which this larva repeats?
Illustration: Figure 225Fig. 225. Two chætopod larvæ.(From Gegenbaur.)o.mouth;i.intestine;a.anus;v.præoral ciliated band;w.perianal ciliated band.
Fig. 225. Two chætopod larvæ.(From Gegenbaur.)o.mouth;i.intestine;a.anus;v.præoral ciliated band;w.perianal ciliated band.
In the first place, this ancestral form, of whichfig. 231A is an ideal representation, would appear to have had a dome-shaped body, with a flattened oral surface and a rounded aboral surface. Its symmetry was radial, and in the centre of the flattened oral surface was placed the mouth, and round its edge was a ring of cilia. The passage of a Pilidium-like larva into the vermiform bilateral Platyelminth form, and therefore it may be presumed of the ancestral form which this larva repeats, is effected by thelarva becoming more elongated, and by the region between the mouth and one end of the body becoming the præoral region, and by an outgrowth between the mouth and the opposite end developing into the trunk, an anus becoming placed at its extremity in the higher forms.
Illustration: Figure 226Fig. 226. Polygordius larva.(After Hatschek.)m.mouth;sg.supraœsophageal ganglion;nph.nephridion;me.p.mesoblastic band;an.anus;ol.stomach.
Fig. 226. Polygordius larva.(After Hatschek.)m.mouth;sg.supraœsophageal ganglion;nph.nephridion;me.p.mesoblastic band;an.anus;ol.stomach.
If what has been so far postulated is correct, it is clear that this primitive larval form bears a very close resemblance to a simplified free-swimming Cœlenterate (Medusa), and that the conversion of such a radiate form into the bilateral took place, not by the elongation of the aboral surface, and the formation of an anus there, but by the unequal elongation of the oral face, an anterior part, together with the dome above it, forming a præoral lobe, and a posterior outgrowth the trunk (figs.226and233); while the aboral surface became the dorsal surface.
This view fits in very well with the anatomical resemblances between the Cœlenterata and the Turbellaria[141], and shews, if true, that the ventral and median position of the mouth in many Turbellaria is the primitive one.
Illustration: Figure 227Fig. 227. Larva of Echiurus.(After Salensky.)m.mouth;an.anus;sg.supraœsophageal ganglion (?).
Fig. 227. Larva of Echiurus.(After Salensky.)m.mouth;an.anus;sg.supraœsophageal ganglion (?).
Illustration: Figure 228Fig. 228. Diagram of a larva of the Polyzoa.m.mouth;an.anus;st.stomach;s.ciliated disc.
Fig. 228. Diagram of a larva of the Polyzoa.m.mouth;an.anus;st.stomach;s.ciliated disc.
The above suggestion as to the mode of passage from the radial into the bilateral form differs largely from that usually held. Lankester[142], for instance, gives the following account of this passage:
≴It has been recognised by various writers, but notably by Gegenbaur and Haeckel, that a condition of radiate symmetry must have preceded the condition of bilateral symmetry in animal evolution. The Diblastula may be conceived to have been at first absolutely spherical with spherical symmetry. The establishment of a mouth led necessarily to the establishment of a structural axis passing through the mouth, around which axis the body was arranged with radial symmetry. This condition is more or less perfectly maintained by many Cœlenterates, and is reassumed by degradationof higher forms (Echinoderms, some Cirrhipedes, some Tunicates). The next step is the differentiation of an upper and a lower surface in relation to the horizontal position, with mouth placed anteriorly, assumed by the organism in locomotion. With the differentiation of a superior and inferior surface, a right and a left side, complementary one to the other, are necessarily also differentiated. Thus the organism becomes bilaterally symmetrical. The Cœlentera are not wanting in indications of this bilateral symmetry, but for all other higher groups of animals it is a fundamental character. Probably the development of a region in front of, and dorsal to the mouth, forming theProstomium, was accomplishedpari passuwith the development of bilateral symmetry. In the radially symmetrical Cœlentera we find very commonly a series of lobes of the body-wall or tentacles producedequally—with radial symmetry, that is to say—all round the mouth, the mouth terminating the main axis of the body—that is to say, the organism being ‘telostomiate.’ The later fundamental form, common to all animals above the Cœlentera, is attained by shifting what was the main axis of the body—so that it may be described now as the ‘enteric’ axis; whilst the new main axis, that parallel with the plane of progression, passes through the dorsal region of the body running obliquely in relation to the enteric axis. Only one lobe or outgrowth of those radially disposed in the telostomiate organisms now persists. This lobe lies dorsally to the mouth, and through it runs the new main axis. This lobe is theProstomium, and all the organisms which thus develop a new main axis, oblique to the old main axis, may be called prostomiate.”
It will be seen from this quotation that the aboral part of the body is supposed to elongate to form the trunk, while the præoral region is derived from one of the tentacles.
Before proceeding to further considerations as to the origin of the Bilateralia, suggested by the Pilidium type of larva, it is necessary to enter into a more detailed comparison between our larval forms.
A very superficial consideration of the characters of these forms brings to light two important features in which they differ,viz.:
(1) In the presence or absence of sense organs on the præoral lobe.
Illustration: Figure 229Fig. 229. Two stages in the development of Tornaria.(After Metschnikoff.)The black lines represent the ciliated bands.m.mouth;an.anus;br.branchial cleft;ht.heart;c.body cavity between splanchnic and somatic mesoblast layers;w.so-called water-vascular vesicle;v.circular blood-vessel.
Fig. 229. Two stages in the development of Tornaria.(After Metschnikoff.)The black lines represent the ciliated bands.m.mouth;an.anus;br.branchial cleft;ht.heart;c.body cavity between splanchnic and somatic mesoblast layers;w.so-called water-vascular vesicle;v.circular blood-vessel.
(2) In the presence or absence of outgrowths from the alimentary tract to form the body cavity.
The larvæ of the Echinodermata and Actinotrocha (?) are without sense organs on the præoral lobe, while the other typesof larvæ are provided with them. Alimentary diverticula are characteristic of the larvæ of the Echinodermata and of Tornaria.
If the conclusion already arrived at to the effect that the prototype of the six larval groups was descended from a radiate ancestor is correct, it appears to follow that the nervous system, in so far as it was differentiated, had primitively a radiate form; and it is also probably true that there were alimentary diverticula in the form of radial pouches,twoof which may have given origin to the paired diverticula which become the body cavity in such types as the Echinodermata, Sagitta, etc. If these two points are granted, the further conclusions seem to follow—(1) that the ganglion and sense organs of the præoral lobe were secondary structures, which arose (perhaps as differentiations of an original circular nerve ring) after the assumption of a bilateral form; and (2) that the absence of these organs in the larvæ of the Echinodermata and Actinotrocha (?) implies that these larvæ retain, so far, more primitive characters than the Pilidium. The same may be said of the alimentary diverticula. There are thus indications that in two important points the Echinoderm larvæ are more primitive than the Pilidium.
Illustration: Figure 230Fig. 230. Actinotrocha.(After Metschnikoff.)m.mouth;an.anus.
Fig. 230. Actinotrocha.(After Metschnikoff.)m.mouth;an.anus.
The above conclusions with reference to the Pilidium and Echinoderm larvæ involve some not inconsiderable difficulties, and suggest certain points for further discussion.
In the first place it is to be noted that the above speculations render it probable that the type of nervous system from which that found in the adults of the Echinodermata, Platyelminthes, Chætopoda, Mollusca, etc., is derived, was a circumoral ring, like that of Medusæ, with which radially arranged sense organs may have been connected; and that in the Echinodermatathis form of nervous system has been retained, while in the other types it has been modified. Its anterior part may have given rise to supraœsophageal ganglia and organs of vision; these beingdeveloped on the assumption of a bilaterally symmetrical form, and the consequent necessity arising for the sense organs to be situated at the anterior end of the body. If this view is correct, the question presents itself as to how far the posterior part of the nervous system of the Bilateralia can be regarded as derived from the primitive radiate ring.
Illustration: Figure 231Fig. 231. Three diagrams representing the ideal evolution of various larval forms.A. Ideal ancestral larval form.B. Larval form from which the Trochosphere larva may have been derived.C. Larval form from which the typical Echinoderm larva may have been derived.m.mouth;an.anus;st.stomach;s.g.supraœsophageal ganglion. The black lines represent the ciliated bands.
Fig. 231. Three diagrams representing the ideal evolution of various larval forms.A. Ideal ancestral larval form.B. Larval form from which the Trochosphere larva may have been derived.C. Larval form from which the typical Echinoderm larva may have been derived.m.mouth;an.anus;st.stomach;s.g.supraœsophageal ganglion. The black lines represent the ciliated bands.
A circumoral nerve-ring, if longitudinally extended, might give rise to a pair of nerve-cords unitedin front and behind—exactly such a nervous system, in fact, as is present in many Nemertines[143](the Enopla and Pelagonemertes), in Peripatus[144], and in primitive molluscan types (Chiton, Fissurella, etc.). From the lateral parts of this ring it would be easy to derive the ventral cord of the Chætopoda and Arthropoda. It is especially deserving of notice in connection with the nervous system of theabove-mentioned Nemertines and Peripatus, that the commissure connecting the two nerve-cords behind is placed on thedorsalside of the intestine. As is at once obvious, by referring to the diagram (fig. 231B), this is the position this commissure ought, undoubtedly, to occupy if derived from part of a nerve-ring which originally followed more or less closely the ciliated edge of the body of the supposed radiate ancestor.
The fact of this arrangement of the nervous system being found in so primitive a type as the Nemertines tends to establish the views for which I am arguing; the absence or imperfect development of the two longitudinal cords in Turbellarians may very probably be due to the posterior part of the nerve-ring having atrophied in this group.
It is by no means certain that this arrangement of the nervous system in some Mollusca and in Peripatus is primitive, though it may be so.
In the larvæ of the Turbellaria the development of sense organs in the præoral region is very clear (fig. 222B); but this is by no means so obvious in the case of the true Pilidium. There is in Pilidium (fig. 232A) a thickening of epiblast at the summit of the dorsal dome, which might seem, from the analogy of Mitraria, etc. (fig. 233), to correspond to the thickening of the præoral lobe, which gives rise to the supraœsophageal ganglion; but, as a matter of fact, this part of the larva does not apparently enter into the formation of the young Nemertine (fig. 232). The peculiar metamorphosis, which takes place in the development of the Nemertine out of the Pilidium[145], may, perhaps, eventually supply an explanation of this fact; but at present it remains as a still unsolved difficulty.
The position of the flagellum in Pilidium, and of the supraœsophageal ganglion in Mitraria, suggests a different view of the origin of the supraœsophageal ganglion from that adopted above. The position of the ganglion in Mitraria corresponds closely with that of the auditory organ in Ctenophora; and it is not impossible that the two structures may have had a common origin. If this view is correct, we must suppose that the apex of the aboral lobe has become the centre of the præoral field of the Pilidium and Trochosphere larval forms[146]—a view which fits in very well with their structure (figs.226and233). The whole of the questions concerning the nervous system are still very obscure, and until further facts are brought to light no definite conclusions can be arrived at.
The absence of sense organs on the præoral lobe of larval Echinodermata, coupled with the structure of the nervous system of the adult, points to the conclusion that the adult Echinodermatahave retained, and not, as is now usually held, secondarily acquired, their radial symmetry; and if this is admitted it follows that the obvious bilateral symmetry of Echinoderm larvæ is a secondary character.
Illustration: Figure 232Fig. 232. A. Pilidium with an advanced Nemertine Worm. B. Ripe embryo of Nemertes in the position it occupies in Pilidium.(Both after Bütschli.)œ.œsophagus;st.stomach;i.intestine;pr.proboscis;lp.lateral pit (cephalic sack);an.amnion;n.nervous system.
Fig. 232. A. Pilidium with an advanced Nemertine Worm. B. Ripe embryo of Nemertes in the position it occupies in Pilidium.(Both after Bütschli.)œ.œsophagus;st.stomach;i.intestine;pr.proboscis;lp.lateral pit (cephalic sack);an.amnion;n.nervous system.
The bilateral symmetry of many Cœlenterate larvæ (the larva of Æginopsis, of many Acraspeda, of Actinia,&c.), coupled with the fact that a bilateral symmetry is obviously advantageousto a free-swimming form, is sufficient to shew that this supposition is by no means extravagant; while the presence of only two alimentary diverticula in Echinoderm larvæ is quite in accord with the presence of a single pair of perigastric chambers in the early larva of Actinia, though it must be admitted that the derivation of the water-vascular system from the left diverticulum is not easy to understand on this view.
A difficulty in the above speculation is presented by the fact of the anus of the Echinodermata being the permanent blastopore, and arising prior to the mouth. If this fact has any special significance, it becomes difficult to regard the larva of Echinoderms and that of the other types as in any way related; but if the views already urged, in a previous section on the germinal layers, as to the unimportance of the blastopore, are admitted, the fact of the anus coinciding with the blastopore ceases to be a difficulty. As may be seen, by referring tofig. 231C, the anus is placed on the dorsal side of the ciliated band. This position for the anus adapts itself to the view that the Echinoderm larva had originally a radial symmetry,with the anus placed at the aboral apex, and that, with the elongation of the larva on the attainment of a bilateral symmetry, the aboral apex became shifted to the present position of the anus.
It may be noticed that the obscure points connected with the absence of a body cavity in most adult Platyelminthes, which have already been dealt with in the section of this chapter devoted to the germinal layers, present themselves again here; and that it is necessary to assume either that alimentary diverticula, like those in the Echinodermata, were primitively present in the Platyelminthes, but have now disappeared from the ontogeny of this group, or that the alimentary diverticula have not become separated from the alimentary tract.
So far the conclusion has been reached that the archetype of the six types of larvæ had a radiate form, and that amongst existing larvæ it is most nearly approached in general shape and in the form of the alimentary canal by the Pilidium group, and in certain other particulars by the Echinoderm larvæ.
The edge of the oral disc of the larval archetype was probably armed with a ciliated ring, from which the ciliated ring of the Pilidium type and of the Echinodermata was most likely derived. The ciliated ring of the Pilidium varies greatly in its characters,and has not always the form of a complete ring. In Pilidium proper (fig. 232A) it is a simple ring surrounding the edge of the oral disc. In Müller’s larva of Thysanozoon (fig. 222B) it is inclined at an axis to the oral disc, and might be called præoral, but such a term cannot be properly used in the absence of an anus.
Illustration: Figure 233Fig. 233. Two stages in the development of Mitraria.(After Metschnikoff.)m.mouth;an.anus;sg.supraœsophageal ganglion;br.andb.provisional bristles;pr.b.præoral ciliated band.
Fig. 233. Two stages in the development of Mitraria.(After Metschnikoff.)m.mouth;an.anus;sg.supraœsophageal ganglion;br.andb.provisional bristles;pr.b.præoral ciliated band.
Illustration: Figure 234Fig. 234. Cyphonautes (larva of Membranipora).(After Hatschek.)m.mouth;a´.anus;f.g.foot gland;x.problematical body (probably a bud). The aboral apex is turned downwards.
Fig. 234. Cyphonautes (larva of Membranipora).(After Hatschek.)m.mouth;a´.anus;f.g.foot gland;x.problematical body (probably a bud). The aboral apex is turned downwards.
The Echinoderm ring is oblique to the axis of the body, and, owing to the fact of its passing ventrally in front of the anus, must be called postoral.
The next point to be considered is that of the affinities of the other larval types to these two types.
The most important of all the larval types is the Trochosphere, and this type is undoubtedly more closely related to the Pilidium than to the Echinoderm larva. Mitraria amongst the Chætopods (fig. 233) has, indeed, nearly the form of a Pilidium, and mainly differs from a Pilidium in the possession of an anus and of provisional bristles; the same may be said of Cyphonautes (fig. 234) amongst the Polyzoa.
The existence of these two forms appears to shew that the præoral ciliated ring of the Trochosphere may very probably be derived directly from the circumoral ciliated ring of the Pilidium; the other ciliated rings or patches of the Trochosphere having a secondary origin.
The larva of the Brachiopoda (fig. 220), in spite of its peculiar characters, is, in all probability, more closely related to the Chætopod Trochosphere than to any other larval type. The most conspicuous point of agreement between them is, however, the possession in common of provisional setæ.
Echinoderm larvæ differ from the Trochosphere, not only in the points already alluded to, but in the character of the ciliated band. The Echinoderm band is longitudinal and postoral. As just stated, there is reason to think that the præoral band of the Trochosphere and the postoral band of the Echinoderm larva are both derived from a ciliated ring surrounding the oral disc of the prototype of these larvæ (videfig. 231). In the case of the Echinodermata the anus must have been formed on thedorsal sideof this ring, and in the case of the Trochosphere on theventral side; and so the difference in position between the two rings was brought about. Another view with reference to these rings has been put forward by Gegenbaur and Lankester, to the effect that the præoral ring of the Trochosphere is derived from the breaking up of the single band of most Echinoderm larvæ into the two bands found in Bipinnaria (videfig. 223) and the atrophy of the posterior band. There is no doubt a good deal to be said for this origin of the præoral ring, and it isstrengthened by the case of Tornaria; but the view adopted above appears to me more probable.
Actinotrocha (fig. 230) undoubtedly resembles more closely Echinoderm larvæ than the Trochosphere. Its ciliated ring has Echinoderm characters, and the growth along the line of the ciliated ring of a series of arms is very similar to what takes place in many Echinoderms. It also agrees with the Echinoderm larvæ in the absence of sense organs on the præoral lobe.
Tornaria (fig. 229) cannot be definitely united either with the Trochosphere or with the Echinoderm larval type. It has important characters in common with both of these groups, and the mixture of these characters renders it a very striking and well-defined larval form.
Phylogenetic conclusions. The phylogenetic conclusions which follow from the above views remain to be dealt with. The fact that all the larvæ of the groups above the Cœlenterata can be reduced to a common type seems to indicate that all the higher groups are descended from a single stem.
Considering that the larvæ of comparatively few groups have persisted, no conclusions as to affinities can be drawn from the absence of a larva in any group; and the presence in two groups of a common larval form may be taken as proving a common descent, but does not necessarily shew any close affinity.
There is every reason to believe that the types with a Trochosphere larva,viz.the Rotifera, the Mollusca, the Chætopoda, the Gephyrea, and the Polyzoa, are descended from a common ancestral form; and it is also fairly certain there was a remote ancestor common to these forms and to the Platyelminthes. A general affinity of the Brachiopoda with the Chætopoda is more than probable. All these types, together with various other types which are nearly related to them, but have not preserved an early larval form, are descended from a bilateral ancestor. The Echinodermata, on the other hand, are probably directly descended from a radial ancestor, and have more or less completely retained their radial symmetry. How far Actinotrocha[147]is related to the Echinoderm larvæ cannot be settled. Its characters may possibly be secondary, like those of themesotrochal larvæ of Chætopods, or they may be due to its having branched off very early from the stock common to the whole of the forms above the Cœlenterata. The position of Tornaria is still more obscure. It is difficult, in the face of the peculiar water-vascular vesicle with a dorsal pore, to avoid the conclusion that it has some affinities with the Echinoderm larvæ. Such affinities would seem, on the lines of speculation adopted in this section, to prove that its affinities to the Trochosphere, striking as they appear to be, are secondary and adaptive. From this conclusion, if justified, it would follow that the Echinodermata and Enteropneusta have a remote ancestor in common, but not that the two groups are in any other way related.
General conclusions and summary. Starting from the demonstrated fact that the larval forms of a number of widely separated types above the Cœlenterata have certain characters in common, it has beenprovisionallyassumed that the characters have been inherited from a common ancestor; and an attempt has been made to determine (1) the characters of the prototype of all these larvæ, and (2) the mutual relations of the larval forms in question. This attempt started with certain more or less plausible suggestions, the truth of which can only be tested by the coherence of the results which follow from them, and their capacity to explain all the facts.
The results arrived at may be summarised as follows:
1. The larval forms above the Cœlenterata may be divided into six groups enumerated on pages370to373.
2. The prototype of all these groups was an organism something like a Medusa, with a radial symmetry. The mouth was placed in the centre of a flattened ventral surface. The aboral surface was dome-shaped. Round the edge of the oral surface was a ciliated ring, and probably a nervous ring provided with sense organs. The alimentary canal was prolonged into two or more diverticula, and there was no anus.
3. The bilaterally symmetrical types were derived from this larval form by the larva becoming oval, and the region in front of the mouth forming a præoral lobe, and that behind the mouth growing out to form the trunk. The aboral dome became the dorsal surface.
On the establishment of a bilateral symmetry the anteriorpart of the nervous ring gave rise (?) to the supraœsophageal ganglia, and the optic organs connected with them; while the posterior part of the nerve-ring formed (?) the ventral nerve-cords. The body cavity was developed from two of the primitive alimentary diverticula.
The usual view that radiate forms have become bilateral by the elongation of the aboral dome into the trunk is probably erroneous.
4. Pilidium is the larval form which most nearly reproduces the characters of the larval prototype in the course of its conversion into a bilateral form.
5. The Trochosphere is a completely differentiated bilateral form, in which an anus has become developed. The præoral ciliated ring of the Trochosphere is probably directly derived from the ciliated ring of Pilidium, which is itself the original ring of the prototype of all these larval forms.
6. Echinoderm larvæ, in the absence of a nerve-ganglion or special organs of sense on the præoral lobe, and in the presence of alimentary diverticula, which give rise to the body cavity, retain some characters of the prototype larva which have been lost in Pilidium. The ciliated ring of Echinoderm larvæ is probably derived directly from that of the prototype by the formation of an anus on the dorsal side of the ring. The anus was very probably originally situated at the aboral apex.
Adult Echinoderms have probably retained the radial symmetry of the forms from which they are descended, their nervous ring being directly derived from the circular nervous ring of their ancestors. They have not, as is usually supposed, secondarily acquired their radial symmetry. The bilateral symmetry of the larva is, on this view, secondary, like that of so many Cœlenterate larvæ.
7. The points of similarity between Tornaria and (1) the Trochosphere and (2) the Echinoderm larvæ are probably adaptive in the one case or the other; and, while there is no difficulty in believing that those to the Trochosphere are adaptive, the presence of a water-vascular vesicle with a dorsal pore renders probable a real affinity with Echinoderm larvæ.
8. It is not possible in the present state of our knowledge to decide how far the resemblances between Actinotrocha and Echinoderm larvæ are adaptive or primary.
Bibliography.
(257)Allen Thomson.British Association Address, 1877.(258)A. Agassiz. “Embryology of the Ctenophoræ.”Mem. Amer. Acad. of Arts and Sciences,Vol.X. 1874.(259)K. E. von Baer.Ueb. Entwicklungsgeschichte d. Thiere.Königsberg, 1828-1837.(260)F. M. Balfour. “A Comparison of the Early Stages in the Development of Vertebrates.”Quart. Journ. of Micr. Sci.,Vol.XV. 1875.(261)C. Claus.Die Typenlehre u. E. Haeckel’s sg. Gastræa-theorie.Wien, 1874.(262)C. Claus.Grundzüge d. Zoologie.Marburg und Leipzig, 1879.(263)A. Dohrn.Der Ursprung d. Wirbelthiere u. d. Princip des Functionswechsels.Leipzig, 1875.(264)C. Gegenbaur.Grundriss d. vergleichenden Anatomie.Leipzig, 1878.Videalso Translation.Elements of Comparative Anatomy.Macmillan & Co., 1878.(265)A. Götte.Entwicklungsgeschichte d. Unke.Leipzig, 1874.(266)E. Haeckel.Studien z. Gastræa-theorie, Jena, 1897; and alsoJenaische Zeitschrift,Vols.VIII.andIX.1874-5.(267)E. Haeckel.Schöpfungsgeschichte.Leipzig.Videalso Translation,The History of Creation. King & Co., London, 1878.(268)E. Haeckel.Anthropogenie.Leipzig.Videalso Translation,Anthropogeny. Kegan Paul & Co., London, 1878.(269)B. Hatschek. “Studien üb. Entwicklungsgeschichte d. Anneliden.”Arbeit. a. d. zool. Instit. d. Univer. Wien.1878.(270)O. andR. Hertwig. “Die Actinien.”Jenaische Zeitschrift,Vols.XIII. andXIV.1879.(271)O. andR. Hertwig.Die Cœlomtheorie.Jena, 1881[148].(272)O. Hertwig.Die Chætognathen.Jena, 1880.(273)R. Hertwig.Ueb. d. Bau d. Ctenophoren.Jena, 1880.(274)T. H. Huxley.The Anatomy of Invertebrated Animals.Churchill, 1877.(274*)T. H. Huxley. “On the Classification of the Animal Kingdom.”Quart. J. of Micr. Science,Vol.XV. 1875.(275)N. Kleinenberg.Hydra, eine anatomisch-entwicklungsgeschichtliche Untersuchung.Leipzig, 1872.(276)A. Kölliker.Entwicklungsgeschichte d. Menschen u. d. höh. Thiere.Leipzig, 1879.(277)A. Kowalevsky. “Embryologische Studien an Würmern u. Arthropoden.”Mém. Acad. Pétersbourg, SeriesVII. Vol.XVI.1871.(278)E. R. Lankester. “On the Germinal Layers of the Embryo as the Basis of the Genealogical Classification of Animals.”Ann. and Mag. of Nat. Hist.1873.(279)E. R. Lankester. “Notes on Embryology and Classification.”Quart. Journ. of Micr. Sci.,Vol.XVII. 1877.(280)E. Metschnikoff. “Zur Entwicklungsgeschichte d. Kalkschwämme.”Zeit. f. wiss. Zool.,Vol.XXIV. 1874.(281)E. Metschnikoff. “Spongiologische Studien.”Zeit. f. wiss. Zool.,Vol.XXXII. 1879.(282)A. S. P. Packard.Life Histories of Animals, including Man, or Outlines of Comparative Embryology.Holt and Co., New York, 1876.(283)C. Rabl. “Ueb. d. Entwick. d. Malermuschel.”Jenaische Zeitsch.,Vol.X. 1876.(284)C. Rabl. “Ueb. d. Entwicklung. d. Tellerschnecke (Planorbis).”Morph. Jahrbuch,Vol.V. 1879.(285)H. Rathke.Abhandlungen z. Bildung und Entwicklungsgesch. d. Menschen u. d. Thiere.Leipzig, 1833.(286)H. Rathke.Ueber die Bildung u. Entwicklungs. d. Flusskrebses.Leipzig, 1829.(287)R. Remak.Untersuch. üb. d. Entwick. d. Wirbelthiere.Berlin, 1855.(288)Salensky. “Bemerkungen üb. Haeckels Gastræa-theorie.”Archiv. f. Naturgeschichte, 1874.(289)E. Schäfer. “Some Teachings of Development.”Quart. Journ. of Micr. Science,Vol.XX. 1880.(290)C. Semper. “Die Verwandtschaftbeziehungen d. gegliederten Thiere.”Arbeiten a. d. zool.-zoot. Instit. Würzburg,Vol.III. 1876-7.