Illustration: Figure 209Fig. 209. Three stages in the development of Sagitta.(A. and C. after Bütschli, and B. after Kowalevsky.)The three embryos are represented in the same positions.A. Represents the gastrula stage.B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three.C. Represents a later stage, in which the mouth involution (m) has become continuous with the alimentary tract, and the blastopore has become closed.m.mouth;al.alimentary canal;ae.archenteron;bl.p.blastopore;pv.perivisceral cavity;sp.splanchnic mesoblast;so.somatic mesoblast;ge.generative organs.
Fig. 209. Three stages in the development of Sagitta.(A. and C. after Bütschli, and B. after Kowalevsky.)The three embryos are represented in the same positions.A. Represents the gastrula stage.B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three.C. Represents a later stage, in which the mouth involution (m) has become continuous with the alimentary tract, and the blastopore has become closed.m.mouth;al.alimentary canal;ae.archenteron;bl.p.blastopore;pv.perivisceral cavity;sp.splanchnic mesoblast;so.somatic mesoblast;ge.generative organs.
Echinodermata.—The lining of the peritoneal cavity is developed from the walls of outgrowths of the archenteron, but the greater part of the mesoblast is derived from the amœboid cells budded off from the walls of the archenteron (fig. 210).
Illustration: Figure 210Fig. 210. Longitudinal section through an embryo of Cucumaria doliolum at the end of the fourth day.Vpv.vaso-peritoneal vesicle;ME.mesenteron;Blp.,Ptd.blastopore, proctodæum.
Fig. 210. Longitudinal section through an embryo of Cucumaria doliolum at the end of the fourth day.Vpv.vaso-peritoneal vesicle;ME.mesenteron;Blp.,Ptd.blastopore, proctodæum.
Enteropneusta (Balanoglossus).—The body cavity is derived from two pairs of alimentary diverticula, the walls of which give rise to the greater part of the mesoblast.
Chordata.—Paired archenteric outgrowths give rise to the whole mesoblast in Amphioxus (fig. 211), and the mode of formation of the mesoblast in other Chordata is probably secondarily derived from this.
Illustration: Figure 211Fig. 211. Sections of an Amphioxus embryo at three stages.(After Kowalevsky.)A. Section at gastrula stage.B. Section of a somewhat older embryo.C. Section through the anterior part of still older embryo.np.neural plate;nc.neural canal;mes.archenteron in A, and mesenteron in B and C;ch.notochord;so.mesoblastic somite.
Fig. 211. Sections of an Amphioxus embryo at three stages.(After Kowalevsky.)A. Section at gastrula stage.B. Section of a somewhat older embryo.C. Section through the anterior part of still older embryo.np.neural plate;nc.neural canal;mes.archenteron in A, and mesenteron in B and C;ch.notochord;so.mesoblastic somite.
3. The cells which will form the mesoblast become marked out very early, and cannot be regarded as definitely springing from either of the primary layers.
Turbellaria.—Leptoplana (fig. 212), Planaria polychroa (?).
Illustration: Figure 212Fig. 212. Sections through the ovum of Leptoplana tremellaris in three stages of development.(After Hallez.)ep.epiblast;m.mesoblast;hy.yolk-cells (hypoblast);bl.blastopore.
Fig. 212. Sections through the ovum of Leptoplana tremellaris in three stages of development.(After Hallez.)ep.epiblast;m.mesoblast;hy.yolk-cells (hypoblast);bl.blastopore.
Chætopoda.—Lumbricus,&c.
Discophora.
It is very possible that the cases quoted under this head ought more properly to belong to group 1.
4. The mesoblast cells are split off from the epiblast.
Nemertea.—Larva of Desor. The mesoblast is stated to be split off from the four invaginated discs.
5. The mesoblast is split off from the hypoblast.
Nemertea.—Some of the types without a metamorphosis.
Mollusca.—Scaphopoda. It is derived from the lateral and ventral cells of the hypoblast.
Gephyrea.—Phascolosoma.
Vertebrata.—In most of the Ichthyopsida the mesoblast is derived from the hypoblast (fig. 213). In some types (i.e.most of the Amniota) the mesoblast might be described as originating at the lips of the blastopore (primitive streak).
Illustration: Figure 213Fig. 213. Two sections of a young Elasmobranch embryo, to shew the mesoblast split off as two lateral masses from the hypoblast.mg.medullary groove;ep.epiblast;m.mesoblast;hy.hypoblast;n.al.cells formed around the nuclei of the yolk which have entered the hypoblast.
Fig. 213. Two sections of a young Elasmobranch embryo, to shew the mesoblast split off as two lateral masses from the hypoblast.mg.medullary groove;ep.epiblast;m.mesoblast;hy.hypoblast;n.al.cells formed around the nuclei of the yolk which have entered the hypoblast.
6. The mesoblast is derived from both germinal layers.
Tracheata.—Araneina (fig. 214). It is derived partly from cells split off from the epiblast and partly from the yolk-cells; but it is probable that the statement that the mesoblast is derived from both the germinal layers is only formally accurate; and that the derivation of part of the mesoblast from the yolk-cells is not to be interpreted as a derivation from the hypoblast.
Illustration: Figure 214Fig. 214. Section through an embryo of Agelena labyrinthica.The section is represented with the ventral plate upwards. In the ventral plate is seen a keel-like thickening, which gives rise to the main mass of the mesoblast.yk.yolk divided into large polygonal cells, in several of which are nuclei.
Fig. 214. Section through an embryo of Agelena labyrinthica.The section is represented with the ventral plate upwards. In the ventral plate is seen a keel-like thickening, which gives rise to the main mass of the mesoblast.yk.yolk divided into large polygonal cells, in several of which are nuclei.
Amniota.—The derivation of the mesoblast of the Amniota from both the primary germinal layers is without doubt a secondary process.
The conclusions to be drawn from the above summary are by no means such as might have been anticipated. The analogy of the Cœlenterata would lead us to expect that the mesoblast would be derived partly from the epiblast and partly from the hypoblast. Such, however, is not for the most part the case, though more complete investigations may shew that there are a greater number of instances in which the mesoblast has a mixed origin than might be supposed from the above summary.
I have attempted to reduce the types of development of the mesoblast to six; but owing to the nature of the case it is not always easy to distinguish the first of these from the last four. Of the six types the second will on most hands be admitted to be the most remarkable. The formation of hollow outgrowths of the archenteron, the cavities of which give rise to the body cavity, can only be explained on the supposition that the body cavity of the types in which such outgrowths occur is derived from diverticula cut off from the alimentary tract. The lining epithelium of the diverticula—the peritoneal epithelium—is clearly part of the primitive hypoblast, and this part of the mesoblast is clearly hypoblastic in origin.
In the case of the Chætognatha (Sagitta), Brachiopoda, and Amphioxus, the whole of the mesoblast originates from the walls of the diverticula; while in the Echinodermata the walls of the diverticula only give rise to the vaso-peritoneal epithelium, the remainder of the mesoblast being derived from amœboid cells which spring from the walls of the archenteron before the origin of the vaso-peritoneal outgrowths (figs.199and210).
Reserving for the moment the question as to what conclusions can be deduced from the above facts as to the origin of the mesoblast, it is important to determine how far the facts of embryology warrant us in supposing that in the whole of the triploblastic forms the body cavity originated from the alimentary diverticula. There can be but little doubt that the mode of origin of the mesoblast in many Vertebrata, as two solid plates split off from the hypoblast, in which a cavity is secondarily developed, is an abbreviation of the process observable in Amphioxus; but this process approaches in some forms ofVertebrata to the ingrowth of the mesoblast from the lips of the blastopore.
It is, therefore, highly probable that the paired ingrowths of the mesoblast from the lips of the blastopore may have been in the first instance derived from a pair of archenteric diverticula. This process of formation of the mesoblast is, as may be seen by reference to the summary, the most frequent, including as it does the Chætopoda, the Mollusca, the Arthropoda,&c.[133]
While there is no difficulty in the view that the body cavity may have originated from a pair of enteric diverticula in the case of the forms where a body cavity is present, there is a considerable difficulty in holding this view, for forms in which there is no body cavity distinct from the alimentary diverticula.
Of these types the Platyelminthes are the most striking. It is, no doubt, possible that a body cavity may have existed in the Platyelminthes, and become lost; and the case of the Discophora, which in their muscular and connective tissue systems as well as in the absence of a body cavity resemble the Platyelminthes, may be cited in favour of this view, in that, being closely related to the Chætopoda, they are almost certainly descended from ancestors with a true body cavity. The usual view of the primitive character of thePlatyelminthes, which has much to support it, is, however, opposed to the idea that the body cavity has disappeared.
If Kowalevsky[134]is right in stating that he has found a form intermediate between the Cœlenterata and the Platyelminthes, there will be strong grounds for holding that the Platyelminthes are, like the Cœlenterata, forms the ancestors of which were not provided with a body cavity.
Perhaps the triploblastica are composed of two groups,viz.(1) a more ancestral group (the Platyelminthes), in which there is no body cavity as distinct from the alimentary, and (2) a group descended from these, in which two of the alimentary diverticula have become separated from the alimentary tract to form a body cavity (remaining triploblastica). However this may be, the above considerations are sufficient to shew how much there is that is still obscure with reference even to the body cavity.
If embryology gives no certain sound as to the questions just raised with reference to the body cavity, still less is it to be hoped that the remaining questions with reference to the origin of the mesoblast can be satisfactorily answered. It is clear, in the first place, from an inspection of the summary given above, that the process of development of the mesoblast is, in all the higher forms, very much abbreviated and modified. Not only is its differentiation relatively deferred, but it does not in most cases originate, as it must have done to start with, as a more orless continuous sheet, split off from parts of one or both the primary layers. It originates in most cases from the hypoblast, and although the considerations already urged preclude us from laying very great stress on this mode of origin, yet the derivation of the mesoblast from the walls of archenteric outgrowths suggests the view that the whole, or at any rate the greater part, of the mesoblast primitively arose by a process of histogenic differentiation from the walls of the archenteron or rather from diverticula of these walls. This view, which was originally put forward by myself (No.260), appears at first sight very improbable, but if the statement of the Hertwigs (No.270), that there is a large development of a hypoblastic muscular system in the Actinozoa, is well founded, it cannot be rejected as impossible. Lankester (No.279), on the other hand, has urged that the mode of origin of the mesoblast in the Echinodermata is more primitive; and that the amœboid cells which here give rise to the muscular and connective tissues represent cells which originally arose from the whole inner surface of the epiblast. It is, however, to be noted that even in the Echinodermata the amœboid cells actually arise from thehypoblast, and their mode of origin may, therefore, be used to support the view that the main part of the muscular system of higher types is derived from the primitive hypoblast.
The great changes which have taken place in the development of the mesoblast would be more intelligible on this view than on the view that the major part of the mesoblast primitively originated from the epiblast. The presence of food-yolk is much more frequent in the hypoblast than in the epiblast; and it is well known that a large number of the changes in early development are caused by food-yolk. If, therefore, the mesoblast has been derived from the hypoblast, many more changes might be expected to have been introduced into its early development than if it had been derived from the epiblast. At the same time the hypoblastic origin of the mesoblast would assist in explaining how it has come about that the development of the nervous system is almost always much less modified than that of the mesoblast, and that the nervous system is not, as might, on the grounds of analogy, have been anticipated, as a rule secondarily developed in the mesoblast.
The Hertwigs have recently suggested in their very interesting memoir (No.271) that the Triploblastica are to be divided into two phyla, (1) the Enterocœla, and (2) the Pseudocœla; the former group containing the Chætopoda, Gephyrea, Brachiopoda, Nematoda, Arthropoda, Echinodermata, Enteropneusta and Chordata; and the latter the Mollusca, Polyzoa, the Rotifera, and Platyelminthes.
The Enterocœla are forms in which the primitive alimentary diverticula have given origin to the body cavity, while the major part of the muscular system has originated from the epithelial walls of these diverticula, part however being in many cases also derived from the amœboid cells, called by them mesenchyme, by the second process of mesoblastic differentiation mentioned onp.347.
In the Pseudocœla the muscular system has become differentiated from mesenchyme cells; while the body cavity, where it exists, is merely a split in the mesenchyme.
It is impossible for me to attempt in this place to state fully, or do justice to, the original and suggestive views contained in this paper. The general conclusion I cannot however accept. The views of the Hertwigs depend to a large extent upon the supposition that it is possible to distinguish histologically muscle cells derived from epithelial cells, from those derived from mesenchyme cells. That in many cases, and strikingly so in the Chordata, the muscle cells retain clear indications of their primitive origin from epithelial cells, I freely admit; but I do not believe either that its histological character can ever be conclusive as to the non-epithelial origin of a muscle cell, or that its derivation in the embryo from an indifferent amœboid cell is any proof that it did not, to start with, originate from an epithelial cell.
I hold, as is clear from the preceding statements, that such immense secondary modifications have taken place in the development of the mesoblast, that no such definite conclusions can be deduced from its mode of development as the Hertwigs suppose.
In support of the view that the early character of embryonic cells is no safe index as to their phylogenetic origin, I would point to the few following facts.
(1) In the Porifera and many of the Cœlenterata (Eucope polystyla, Geryonia,&c.) the hypoblast (endoderm) originates from cells, which according to the Hertwigs’ views ought to be classed as mesenchyme.
(2) In numerous instances muscles which have, phylogenetically, an undoubted epithelial origin, are ontogenetically derived from cells which ought to be classed as mesenchyme. The muscles of the head in all the higher Vertebrata, in which the head cavities have disappeared, are examples of this kind; the muscles of many of the Tracheata, notably the Araneina, must also be placed in the same category.
(3) The Mollusca are considered by the Hertwigs to be typical Pseudocœla. A critical examination of the early development of the mesoblast in these forms demonstrates however that with reference to the mesoblast theymust be classed in the same group as the Chætopoda. The mesoblast (Vol.II. p.227) clearly originates as two bands of cells which grow inwards from the blastopore, and in some forms (Paludina,Vol.II. fig. 107) become divided into a splanchnic and somatic layer, with a body cavity between them. All these processes are such as are, in other instances, admitted to indicate Enterocœlous affinities.
The subsequent conversion of the mesoblast elements into amœboid cells, out of which branched muscles are formed, is in my opinion simply due to the envelopment of the soft Molluscan body within a hard shell.
In addition to these instances I may point out that the distinction between the Pseudocœla and Enterocœla utterly breaks down in the case of the Discophora, and the Hertwigs have made no serious attempt to discuss the characters of this group in the light of their theory, and that the derivation of the Echinoderm muscles from mesenchyme cells is a difficulty which is very slightly treated.
II.Larval forms: their nature, origin and affinities.
Preliminary considerations. In a general way two types of development may be distinguished,viz.a fœtal type and a larval type. In the fœtal type animals undergo the whole or nearly the whole of their development within the egg or within the body of the parent, and are hatched in a condition closely resembling the adult; and in the larval type they are born at an earlier stage of development, in a condition differing to a greater or less extent from the adult, and reach the adult state either by a series of small steps, or by a more or less considerable metamorphosis.
The satisfactory application of embryological data to morphology depends upon a knowledge of the extent to which the record of ancestral history has been preserved in development. Unless secondary changes intervened this record would be complete; it becomes therefore of the first importance to the embryologist to study the nature and extent of the secondary changes likely to occur in the fœtal or the larval state.
The principles which govern the perpetuation of variations which occur in either the larval or the fœtal state are the same as those for the adult condition. Variations favourable to the survival of the species are equally likely to be perpetuated, at whatever period of life they occur, prior to the loss of the reproductive powers. The possible nature and extent of thesecondary changes which may have occurred in the developmental history of forms, which have either a long larval existence, or which are born in a nearly complete condition, is primarily determined by the nature of the favourable variations which can occur in each case.
Where the development is a fœtal one, the favourable variations which can most easily occur are—(1) abbreviations, (2) an increase in the amount of food-yolk stored up for the use of the developing embryo. Abbreviations take place because direct development is always simpler, and therefore more advantageous; and, owing to the fact of the fœtus not being required to lead an independent existence till birth, and of its being in the meantime nourished by food-yolk, or directly by the parent, there are no physiological causes to prevent the characters of any stage of the development,which are of functional importance during a free but not during a fœtal existence, from disappearing from the developmental history. All organs of locomotion and nutrition not required by the adult will, for this reason, obviously have a tendency to disappear or to be reduced in fœtal developments; and a little consideration will shew that the ancestral stages in the development of the nervous and muscular systems, organs of sense, and digestive system will be liable to drop out or be modified,when a simplification can thereby be effected. The circulatory and excretory systems will not be modified to the same extent, because both of them are usually functional during fœtal life.
The mechanical effects of food-yolk are very considerable, and numerous instances of its influence will be found in the earlier chapters of this work[135]. It mainly affects the early stages of development,i.e.the form of the gastrula,&c.
The favourable variations which may occur in the free larva are much less limited than those which can occur in the fœtus. Secondary characters are therefore very numerous in larvæ, and there may even be larvæ with secondary characters only, as, for instance, the larvæ of Insects.
In spite of the liability of larvæ to acquire secondary characters, there is a powerful counterbalancing influence tendingtowards the preservation of ancestral characters, in that larvæ are necessarily compelled at all stages of their growth to retainin a functional statesuch systems of organs, at any rate, as are essential for a free and independent existence. It thus comes about that, in spite of the many causes tending to produce secondary changes in larvæ, there is always a better chance of larvæ repeating, in an unabbreviated form, their ancestral history, than is the case with embryos, which undergo their development within the egg.
It may be further noted as a fact which favours the relative retention by larvæ of ancestral characters, that a secondary larval stage is less likely to be repeated in development than an ancestral stage, because there is always a strong tendency for the former, which is a secondarily intercalated link in the chain of development, to drop out by the occurrence of areversionto the original type of development.
The relative chances of the ancestral history being preserved in the fœtus or the larva may be summed up in the following way:—There is a greater chance of the ancestral history beinglostin forms which develop in the egg; and of its beingmaskedin those which are hatched as larvæ.
The evidence from existing forms undoubtedly confirms thea prioriconsiderations just urged[136]. This is well shewn by a study of the development of Echinodermata, Nemertea, Mollusca, Crustacea, and Tunicata. The free larvæ of the four first groups are more similar amongst themselves than the embryos which develop directly, and since this similarity cannot be supposed to be due to the larvæ having been modified by living under precisely similar conditions, it must be due to their retaining common ancestral characters. In the case of theTunicatathe free larvæ retain much more completely than the embryos certain characters such as the notochord, the cerebrospinal canal, etc., which are known to be ancestral.
Types of Larvæ.—Although there is no reason to suppose that all larval forms are ancestral, yet it seems reasonable to anticipate that a certain number of the known types of larvæ would retain the characters of the ancestors of the more important phyla of the animal kingdom.
Before examining in detail the claims of various larvæ to such a character, it is necessary to consider somewhat more at length the kind of variations which are most likely to occur in larval forms.
It is probablea priorithat there are two kinds of larvæ, which may be distinguished as primary and secondary larvæ. Primary larvæ are more or less modified ancestral forms, which have continued uninterruptedly to develop as free larvæ from the time when they constituted the adult form of the species. Secondary larvæ are those which have become introduced into the ontogeny of species, the young of which were originally hatched with all the characters of the adult; such secondary larvæ may have originated from a diminution of food-yolk in the egg and a consequently earlier commencement of a free existence, or from a simple adaptive modification in the just hatched young. Secondary larval forms may resemble the primary larval forms in cases where the ancestral characters were retained by the embryo in its development within the egg; but in other instances their characters are probably entirely adaptive.
Causes tending to produce secondary changes in larvæ.—The modes of action of natural selection on larvæ may probably be divided more or less artificially into two classes.1. The changes in development directly produced by the existence of a larval stage.2. The adaptive changes in a larva acquired in the ordinary course of the struggle for existence.
The changes which come under the first head consist essentially in a displacement in the order of development of certain organs. There is always a tendency in development to throw back the differentiation of the embryonic cells into definite tissues to as late a date as possible. This takes place in order to enable the changes of form, which every organ undergoes, in repeating even in an abbreviated way its phylogenetic history, to be effected with the least expenditure of energy. Owing tothis tendency it comes about that when an organism is hatched as a larva many of the organs are still in an undifferentiated state, although the ancestral form which this larva represents had all its organs fully differentiated. In order, however, that the larva may be enabled to exist as an independent organism, certain sets of organs,e.g.the muscular, nervous, and digestive systems, have to be histologically differentiated. If the period of fœtal life is shortened, an earlier differentiation of certain organs is a necessary consequence; and in almost all cases the existence of a larval stage causes a displacement in order of development of organs, the complete differentiation of many organs being retarded relatively to the muscular, nervous, and digestive systems.
The possible changes under the second head appear to be unlimited. There is, so far as I see, no possible reason why an indefinite number of organs should not be developed in larvæ to protect them from their enemies, and to enable them to compete with larvæ of other species, and so on. The only limit to such development appears to be the shortness of larval life, which is not likely to be prolonged, since,ceteris paribus, the more quickly maturity is reached the better it is for the species.
A very superficial examination of marine larvæ shews that there are certain peculiarities common to most of them, and it is important to determine how far such peculiarities are to be regarded as adaptive. Almost all marine larvæ are provided with well-developed organs of locomotion, and transparent bodies. These two features are precisely those which it is most essential for such larvæ to have. Organs of locomotion are important, in order that larvæ may be scattered as widely as possible, and so disseminate the species; and transparency is very important in rendering larvæ invisible, and so less liable to be preyed upon by their numerous enemies[137].
These considerations, coupled with the fact that almost all free-swimming animals, which have not other special means of protection, are transparent, seem to shew that the transparencyof larvæ at all events is adaptive; and it is probable that organs of locomotion are in many cases specially developed, and not ancestral.
Various spinous processes on the larvæ of Crustacea and Teleostei are also examples of secondarily acquired protective organs.
These general considerations are sufficient to form a basis for the discussion of the characters of the known types of larvæ.
The following table contains a list of the more important of such larval forms:Dicyemidæ.—The Infusoriform larva (vol.II.fig.62).Porifera.—(a) The Amphiblastula larva (fig. 215), with one-half of the body ciliated, and the other half without cilia; (b) an oval uniformly ciliated larva, which may be either solid or have the form of a vesicle.Cœlenerata.—The planula (fig. 216).Turbellaria.—(a) The eight-lobed larva of Müller (fig. 222); (b) the larva of Götte and Metschnikoff, with some Pilidium characters.Nemertea.—The Pilidium (fig. 221).Trematoda.—The Cercaria.Rotifera.—The Trochosphere-like larvæ of Brachionus (fig. 217) and Lacinularia.Mollusca.—Mollusca.—The Trochosphere larva (fig. 218), and the subsequent Veliger larva (fig. 219).Brachiopoda.—The three-lobed larva, with a postoral ring of cilia (fig. 220).Polyzoa.—A larval form with a single ciliated ring surrounding the mouth, and an aboral ciliated ring or disc (fig. 228).Chætopoda.—Various larval forms with many characters like those of the molluscan Trochosphere, frequently with distinct transverse bands of cilia. They are classified as Atrochæ, Mesotrochæ, Telotrochæ (fig. 225Aandfig. 226), Polytrochæ, and Monotrochæ (fig. 225B).Gephyrea Nuda.—Larval forms like those of preceding groups. A specially characteristic larva is that of Echiurus (fig. 227).Gephyrea Tubicola.—Actinotrocha (fig. 230), with a postoral ciliated ring of arms.Myriapoda.—A functionally hexapodous larval form is common to all the Chilognatha (vol.II.fig.174).Insecta.—Various secondary larval forms.Crustacea.—The Nauplius (vol.II.fig.208) and the Zoæa (vol.II.fig.210).Echinodermata.—The Auricularia (fig. 223A), the Bipinnaria (fig. 223B), and the Pluteus (fig. 224), and the transversely-ringed larvæ of Crinoidea (vol.II.fig.268). The three first of which can be reduced to a common type (fig. 231C).Enteropneusta.—Tornaria (fig. 229).Urochorda (Tunicata).—The tadpole-like larva (vol.III.fig. 8).Ganoidei.—A larva with a disc with adhesive papillæ in front of the mouth (vol.III.fig. 67).Anurus Amphibia.—The tadpole (vol.III.fig. 80).
Illustration: Figure 215Fig. 215. Two free stages in the development of Sycandra raphanus.(After Schultze.)A. Amphiblastula stage.B. Stage after the ciliated cells have commenced to be invaginated.c.s.segmentation cavity;ec.granular epiblast cells;en.ciliated hypoblast cells.
Fig. 215. Two free stages in the development of Sycandra raphanus.(After Schultze.)A. Amphiblastula stage.B. Stage after the ciliated cells have commenced to be invaginated.c.s.segmentation cavity;ec.granular epiblast cells;en.ciliated hypoblast cells.
Illustration: Figure 216Fig. 216. Three larval stages of Eucope ploystyla.(After Kowalevsky.)A. Blastosphere stage with hypoblast spheres becoming budded into the central cavity.B. Planula stage with solid hypoblast.C. Planula stage with a gastric cavity.ep.epiblast;hy.hypoblast;al.gastric cavity.
Fig. 216. Three larval stages of Eucope ploystyla.(After Kowalevsky.)A. Blastosphere stage with hypoblast spheres becoming budded into the central cavity.B. Planula stage with solid hypoblast.C. Planula stage with a gastric cavity.ep.epiblast;hy.hypoblast;al.gastric cavity.
Of the larval forms included in the above list a certain number are probably without affinities outside the group to which they belong. This is the case with the larvæ of theMyriapoda, the Crustacean larvæ, and with the larval forms of the Chordata. I shall leave these forms out of consideration.
There are, again, some larval forms which may possibly turn out hereafter to be of importance, but from which, in the present state of our knowledge, we cannot draw any conclusions. The infusoriform larva of the Dicyemidæ, and the Cercaria of the Trematodes, are such forms.
Excluding these and certain other forms, we have finally left for consideration the larvæ of the Cœlenterata, the Turbellaria, the Rotifera, the Nemertea, the Mollusca, the Polyzoa, the Brachiopoda, the Chætopoda, the Gephyrea, the Echinodermata, and the Enteropneusta.
The larvæ of these forms can be divided into two groups. The one group contains the larva of the Cœlenterata or Planula, the other group the larvæ of all the other forms.
Illustration: Figure 217Fig. 217. Embryo of Brachionus urceolaris, shortly before it is hatched.(After Salensky.)m.mouth;ms.masticatory apparatus;me.mesenteron;an.anus;ld.lateral gland;ov.ovary;t.tail (foot);tr.trochal disc;sg.supraœsophageal ganglion.
Fig. 217. Embryo of Brachionus urceolaris, shortly before it is hatched.(After Salensky.)m.mouth;ms.masticatory apparatus;me.mesenteron;an.anus;ld.lateral gland;ov.ovary;t.tail (foot);tr.trochal disc;sg.supraœsophageal ganglion.
The Planula (fig. 216) is characterised by its extreme simplicity. It is a two-layered organism, with a form varying from cylindrical to oval, and usually a radial symmetry. So long as it remains free it is not usually provided with a mouth, and it is as yet uncertain whether or no the absence of a mouth is to be regarded as an ancestral character. The Planula is very probably the ancestral form of the Cœlenterata.
The larvæ of almost all the other groups, although they may be subdivided into a series of very distinct types, yet agree in the possession of certain common characters[138]. There is a more or less dome-shaped dorsal surface, and a flattened or concave ventral surface, containing the openingof the mouth, and usually extending posteriorly to the opening of the anus, when such is present.
The dorsal dome is continued in front of the mouth to form alarge præoral lobe.
There is usually present at first an uniform covering of cilia; but in the later larval stages there are almost always formed definite bands or rings of long cilia, by which locomotion is effected. These bands are often produced into arm-like processes.
The alimentary canal has, typically, the form of a bent tube with a ventral concavity, constituted (when an anus is present) of three sections,viz.an œsophagus, a stomach, and a rectum. The œsophagus and sometimes the rectum are epiblastic in origin, while the stomach always and the rectum usually are derived from the hypoblast[139].
Illustration: Figure 218Fig. 218. Diagram of an embryo of Pleurobranchidium.(From Lankester.)f.foot;ot.otocyst;m.mouth;v.velum;ng.nerve ganglion;ry.residual yolk spheres;shs.shell-gland;i.intestine.
Fig. 218. Diagram of an embryo of Pleurobranchidium.(From Lankester.)f.foot;ot.otocyst;m.mouth;v.velum;ng.nerve ganglion;ry.residual yolk spheres;shs.shell-gland;i.intestine.
To the above characters may be added a glass-like transparency; and the presence of a widish space possibly filled with gelatinous tissue, and often traversed by contractile cells, between the alimentary tract and the body wall.
Illustration: Figure 219Fig. 219. Larvæ of Cephalophorous Mollusca in the veliger stage.(From Gegenbaur.)A. and B. Earlier and later stage of Gasteropod. C. Pteropod (Cymbulia).v.velum;c.shell;p.foot;op.operculum;t.tentacle.
Fig. 219. Larvæ of Cephalophorous Mollusca in the veliger stage.(From Gegenbaur.)A. and B. Earlier and later stage of Gasteropod. C. Pteropod (Cymbulia).v.velum;c.shell;p.foot;op.operculum;t.tentacle.
Considering the very profound differences which exist between many of these larvæ, it may seem that the characters just enumerated are hardly sufficient to justify my grouping them together. It is, however, to be borne in mind that my grounds for doing so depend quite as much upon the fact that they constitute a series without any great breaks in it, as upon the existence of characters common to the whole of them. It is also worth noting that most of the characters which have been enumerated as common to the whole of these larvæ are not such secondary characters as (in accordance with the considerations used above) might be expected to arise from the fact of their being subjected to nearly similar conditions of life. Their transparency is, no doubt, such a secondary character, and it is not impossible that the existence of ciliated bands may be so also; but it is quite possible that if, as I suppose, these larvæ reproduce the characters of some ancestral form, this form may have existed at a time when all marine animals were free-swimming, and that it may, therefore, have been provided with at least one ciliated band.
Illustration: Figure 220Fig. 220. Larva of Argiope.(From Gegenbaur; after Kowalevsky.)m.mantle;b.setæ;d.archenteron.
Fig. 220. Larva of Argiope.(From Gegenbaur; after Kowalevsky.)m.mantle;b.setæ;d.archenteron.
The detailed consideration of the characters of these larvæ, given below, supports this view.
This great class of larvæ may, as already stated, be divided into a series of minor subdivisions. These subdivisions are the following:
1. The Pilidium Group.—This group is characterised by the mouth being situated nearly in the centre of the ventral surface, and by the absence of an anus. It includes the Pilidium of the Nemertines (fig. 221), and the various larvæ of marine Dendrocœla (fig. 222). At the apex of the præoral lobe a thickening of epiblast may be present, from which (fig. 232) a contractile cord sometimes passes to the œsophagus.