Read Ebook: The Works of Francis Maitland Balfour Volume 2 (of 4) A Treatise on Comparative Embryology: Invertebrata by Balfour Francis M Francis Maitland Foster M Michael Sir Editor Sedgwick Adam Editor

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In Pleurobrachia there is at first only one otolith at each corner. The otoliths are gradually transported towards the centre of the vesicle and are there attached, though the four leaf-like suspenders do not arise till very late. The otoliths go on increasing in number throughout life.

The gelatinous tissue of the Ctenophora appears as a homogeneous layer between the epiblast and the yolk cells, and is probably homologous with the layer formed in the same situation in all other coelenterate forms. Into the layer a number of anastomosing cells, mainly derived from the epiblast, though according to Chun also in part from the hypoblast, make their way. These cells would appear to be mainly, if not entirely , of a contractile nature. It is probable that the great mass of the gelatinous tissue of the adult is an intercellular substance derived from these cells.

The whole of the above changes are completed while the embryo is still enclosed in the egg-capsule. During their accomplishment the oro-anal axis, which was originally very short, increases greatly in length , so that the embryo acquires an oval form similar to that of the adult.

The exact period of leaving the egg does not appear to be very constant but the hatching never takes place till the embryo has practically acquired all the organs of the adult.

In the majority of types the differences between the just hatched larva and the adult are inconsiderable, and in all cases the larva has a somewhat oval form. In the case of the Taeniatae , the larva has the characteristic oval form, and the subsequent changes amount almost to a metamorphosis.

The larva of the Lobatae, such as Eucharis, Bolina, etc., can hardly be distinguished from Pleurobrachia, and undergoes therefore considerable changes after hatching.

The new genus Ctenaria recently described by Haeckel, which is intermediate between the Ctenophora and the Medusae clearly proves that the Ctenophora are more closely related to the Medusae than to the Actinozoa but their development, especially the presence of a stomodaeum, shews that they have affinities with the Acraspedote as well as with the Craspedote Medusae; and it may be noted that the Acraspeda have undoubted affinities with the Actinozoa.

Even in the adult condition the lower forms of Coelenterata do not rise in complexity much beyond a typical gastrula. Ontogeny nevertheless brings clearly to light the existence of a larval form--the planula--which recurs with fair constancy amongst all the groups except the Ctenophora.

We are probably justified in assuming that the planula is a repetition of a free ancestral form of the Coelenterata. The planula, as it most frequently occurs, is a two-layered ciliated nearly cylindrical organism, with at most a rudimentary digestive cavity hollowed out in the inner layer, and as a rule no mouth. In the outer layer are numerous thread-cells.

How many of these characters did the ancestral planula possess? I think it is not unreasonable to assume that the only two characters about which there can be much doubt are the rudimentary condition of the digestive cavity and the absence of a mouth. Paradoxical as it may seem, it appears to me not impossible that the Coelenterata may have had an ancestor in which a digestive tract was physiologically replaced by a solid mass of amoeboid cells. This ancestor was perhaps common to the Turbellarians also. The constant presence of thread-cells in the inner layer of their epiblast fits in with their derivation from a form similar to the planula. While the solid parenchymatous digestive canal of Convoluta and Schizoprora and other forms amongst the Turbellarians, though very probably secondary, may perhaps be explained by such a view of their origin.

The planula in its primitive condition is not bilaterally symmetrical, but frequently, as amongst the Actinozoa, it becomes flattened on two sides before undergoing its conversion into the adult form. Perhaps the bilateral form of planula is the starting point both for the Coelenterata and the Turbellaria. In this connection the peculiar unilateral development of a tentacle in Scyphistoma and Actinia should be noted.

The planula occurs in the majority of sessile forms of Hydrozoa except the Tubularidae and Hydra. It is also characteristic of the Trachymedusae and Siphonophora. Amongst the Acraspeda it is also present, but has an exceptional mode of ontogeny which is discussed in connection with the germinal layers.

It is characteristic both of the Octocoralla and Hexacoralla, but is not found in the Ctenophora.

Delamination is constant amongst the Hydromedusae and Siphonophora. It is perhaps in the main characteristic of the Actinozoa.

Invagination by embole takes place, so far as is known, constantly amongst the Acraspeda and frequently amongst the Actinozoa; and an epibolic invagination is characteristic of the Ctenophora.

If confidence is to be placed in the recorded observations on which this summary is founded, and there is no reason why in a general way it should not be so placed, the conclusion is inevitable that of the above modes of development the one must be primitive and the other a derivative from it, for, if this conclusion be not accepted, the absolutely inadmissible hypothesis of a double origin for the Coelenterata would have to be adopted.

Two questions arise from these considerations:--

Which is the primitive, delamination or invagination?

How is the one of these to be derived from the other?

There is a great deal to be said in favour of both delamination and invagination; but it will be convenient to defer all discussion of the question to the general chapter on the formation of the layers throughout the animal kingdom.

The hypoblast cells are often filled with yolk material, and secondary modifications are thus produced in the development. The most important examples of such modifications are found in the Siphonophora and Ctenophora.

In the simplest forms amongst the Hydrozoa there is no trace of a third layer or mesoblast. The epiblast is typically formed, as was first shewn by Kleinenberg, of an epithelial layer and a subepithelial interstitial layer of cells. The cells of the former are frequently produced into muscular or nervous tails, and those of the latter give rise to the thread-cells and generative organs and in some cases to muscles. In many cases, amongst all the Coelenterate groups, and constantly amongst the Ctenophora the epiblast is simplified and reduced to a single layer. The hypoblast undergoes in most cases no such differentiation but simply forms a glandular layer lining the gastric chamber and its prolongations into the tentacles; but in the Actinozoa it appears to give rise to muscles, and strong evidence has been brought forward to shew that in some groups it gives rise to the generative organs.

The questions relating to the generative organs of the Coelenterata are dealt with in the second part of this work.

Between the epiblast and hypoblast a structureless lamella appears always to be interposed.

In many Coelenterata further differentiations of the epiblast are present. In many forms the layer gives rise to a hard external skeleton. This is most widely spread amongst the Hydrozoa, where in the majority of cases it takes the form of the horny perisarc, and in the Hydrocoralla of a hard calcareous skeleton. The skeleton in these forms, though closely resembling the mesoblastic skeleton of the Actinozoa, has been shewn by Moseley to be epiblastic.

In the Actinozoa an epiblastic skeleton is exceptional, and according to most authorities absent. Quite recently however Koch has found that the axial branched skeleton of most of the Gorgonidae, viz. the Gorgoninae and Isidinae, is separated from the coenosarc by an epithelium, which he believes to be epiblastic, and to which no doubt the axial skeleton owes its origin. A similar epithelium surrounds the axis of the Pennatulidae.

In the Medusae the epiblast also gives rise to a central nervous system, which however continues to form a constituent part of the layer, and to the organs of special sense.

The differentiation of the nervous and muscular systems in the Hydrozoa is treated of in the second part of this work.

A special differentiation of the hypoblast is found in the solid axis of the tentacles. This axis replaces the gastric prolongation found in many forms, and the cells composing it differentiate themselves into a chorda-like tissue, which has a skeletal function, and is no longer connected with nutrition. This axis is placed by many morphologists amongst the mesoblastic structures.

In all the higher Coelenterata certain tissues become interposed between the epiblast and hypoblast, which may be classified together as the mesoblast.

The most important of these are:

The various distinct muscular layers. The gelatinous tissue of the Medusae and Ctenophora. The skeletogenous tissue of the Actinozoa.

The origin of the gelatinous tissue is still involved in much obscurity.

It originates as a homogeneous layer between epiblast and hypoblast, which in the Hydromedusae never becomes cellular though traversed by elastic fibres.

Alternation of generations is of common occurrence amongst the Hydrozoa, and something analogous to it has been found to take place in Fungia amongst the Actinozoa. It is not known to occur in the Ctenophora.

The chief interest of its occurrence amongst the Hydromedusae and Siphonophora is the fact that its origin can be traced to a division of labour in the colonial systems of zooids so characteristic of these types.

In the Hydromedusae an interesting series of relations between alternation of generations and the division of the zooids into gonophores and trophosomes can be made out. In Hydra the generative and nutritive functions are united in the same individual. The generative swellings in these forms cannot, as has been ably argued by Kleinenberg, be regarded as rudimentary gonophores, but are to be compared to the generative bands developed in the Medusae around parts of the gastro-vascular system. A condition like that of Hydra, in which the ovum directly gives rise to a form like its parent, is no doubt the primitive one, though it is not so certain that Hydra itself is a primitive form. The relation of Hydra to the Tubularidae and Campanularidae may best be conceived by supposing that in Hydra most ordinary buds did not become detached, so that a compound Hydra became formed; but that at certain periods particular buds retained their primitive capacity of becoming detached and subsequently developed generative organs, while the ordinary buds lost their generative function.

It would obviously be advantageous for the species that the detached buds with generative organs should be locomotive, so as to distribute the species as widely as possible, and such buds in connection with their free existence would naturally acquire a higher organization than the attached trophosomes. It is easy to see how, by a series of steps such as I have sketched out, a division of labour might take place, and it is obvious that the embryos produced by the highly organized gonophores would give rise to a fixed form from which the fixed colony would be budded. Thus an alternation of generations would be established as a necessary sequel to such a division of labour. To test the above explanation it is necessary to review the main facts with reference to alternations of generations amongst the Hydromedusae.

Hydromedusae. In many instances amongst the Tubularidae, Sertularidae and Campanularidae medusiform buds are produced which become detached and develop sexual organs.

For a full account of this subject the reader is referred to the beautiful memoir of Allman .

Such Medusae are divided into two great groups, the Ocellata and Vesiculata, according to the characters of the marginal sense organs. In the Ocellata the sense organs have the form of eyes, and in the Vesiculata of auditory vesicles. The latter seem to be usually budded off from the Campanularia stocks, and the generative organs extend in folded bands over the radial canals. These bands have been regarded by Allman as composed of rudimentary gonophores, and he called the Medusae which give rise to them blastochemes. He regards them as representing a more complicated type of alternation of generations with three instead of two generations in the series. The Hertwigs have brought what appear to me conclusive grounds for rejecting this view, and have demonstrated that the generative organs of these types resemble those of ordinary Medusae.

In many forms the medusiform buds though fully developed do not become detached; whether detached or not they are known as phanerocodonic gonophores. In other forms again buds which begin as if they were going to form Medusae never reach that condition but remain permanently in an undeveloped state. They have been called by Allman adelocodonic gonophores.

In all the above cases two generations at the least interpose between the successive sexual periods, viz.:--

A trophosome produced directly from the ovum. A gonophore budded from this.

In a very large number of types the gonophores do not develop directly on the hydroid stem, but arise on specially modified zooids resembling rudimentary trophosomes which have been named blastostyles by Allman. On the sides of each blastostyle a series of gonophores usually becomes developed. The blastostyles either remain exposed as in all the Gymnoblastic or Tubularian Hydroids, or as in all the Calyptoblastic Hydroids they become invested by a special case--known as the gonangium--which is formed of perisarc lined by epiblast. In the forms with blastostyles three generations interpose between the successive stages of sexual reproduction, the trophosome developed directly from the ovum, the blastostyle budded from this, the gonophore budded from the blastostyle.

The Trachymedusae, as has been shewn above, develop directly. They are probably derived from gonophores in which the trophosome has disappeared from the developmental cycle.

To sum up, three types of development are found amongst the Hydromedusae.

Siphonophora. In the Siphonophora alternations of generations take place in the same way as in the Hydromedusae, but the starting point appears to be a Medusa. The gonophores may remain fixed or become detached.

Acraspeda. With the exception of Pelagia and Lucernaria, in which the development involves a simple metamorphosis, all the Acraspeda undergo a form of alternations of generations. The ovum, as already described, develops into a fixed form--the Scyphistoma--which increases asexually by normal budding, and can even form a permanent colony.

The formation of the sexual Medusa form takes place by a kind of strobilization of the body of the fixed Scyphistoma. A series of transverse constrictions becomes formed round the body below the mouth, dividing it up into corresponding rings, each of which eventually gives rise to a Medusa known as an Ephyra . In each of these rings is a dilation of the stomach, and a section of each of the four rudimentary mesenteries described in connection with the development of the Scyphistoma. As the constrictions become deeper the segments of the body between them become disc-like, and their edges are produced into eight lobes containing prolongations of the gastric cavity . The lower surface of each disc, which forms the future aboral surface of the Medusa, becomes convex, in part owing to the development of gelatinous tissue. On the opposite surface a muscular layer becomes developed. During the above process the body of the Scyphistoma gradually grows in length and continues to be segmented, so that a series of Ephyrae are uninterruptedly formed, of which those near the base are the youngest. The original terminal ring of tentacles of the Scyphistoma gradually atrophies.

In the further development of the Ephyrae each of their eight lobes becomes bifid at its extremity.

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