Larva of Sycandra raphanusFig. 64. Larva of Sycandra raphanus at pseudogastrula stage, in situ in the maternal tissues.(Copied from F. E. Schulze.)me.mesoblast of adult;hy.collared cells forming hypoblast of the adult;en.clear cells of larva which eventually become involuted to form the hypoblast;ec.granular cells of larva which give rise to the epiblast, which at this stage are partially involuted.
Fig. 64. Larva of Sycandra raphanus at pseudogastrula stage, in situ in the maternal tissues.(Copied from F. E. Schulze.)
me.mesoblast of adult;hy.collared cells forming hypoblast of the adult;en.clear cells of larva which eventually become involuted to form the hypoblast;ec.granular cells of larva which give rise to the epiblast, which at this stage are partially involuted.
This stage nearly completes the segmentation: in the next one, the cells of the poles of the blastosphere increase in number,and the cells of the greater part of the blastosphere become columnar and ciliated, (fig. 64en.) while the granular cells (ec.) increase to about thirty-two in number and appear to be (partially at least) involuted into the segmentation cavity, reducing this latter to a mere slit. This stage forms the last passed by the embryo in the tissues of the parent. The general position of the embryo while still in this situation may be gathered fromfig. 64, representing the embryoin situ. The embryo is always placed close to one of the radial canals. From this situation it makes its way through the lining cells into a canal and is thence transported to the surrounding water. By the time the larva has become free, the semi-invaginated granular cells have increased in bulk and become everted so as to project very much more prominently than in the encapsuled state. To the gastrula stage, if it deserves the name, passed through by the embryo in the tissues of the parent, no importance can be attached.
Development of Sycandra raphanusFig. 65. Two free stages in the development of Sycandra raphanus.(Copied from Schulze.)A. Amphiblastula stage.B. A later stage after the ciliated cells have commenced to become invaginated.cs.segmentation cavity;ec.granular cells which will form the epiblast;en.ciliated cells which become invaginated to form the hypoblast.
Fig. 65. Two free stages in the development of Sycandra raphanus.(Copied from Schulze.)
A. Amphiblastula stage.B. A later stage after the ciliated cells have commenced to become invaginated.
cs.segmentation cavity;ec.granular cells which will form the epiblast;en.ciliated cells which become invaginated to form the hypoblast.
The larva, after it has left the parental tissues, has an oval form and is transversely divided into two areas (fig. 65A). One of these areas is formed of the elongated, clear, ciliated cells, with a small amount of pigment near their inner ends (en.), andthe other and larger area of the thirty-two granular cells already mentioned (ec.). Fifteen or sixteen of these are arranged as a special ring on the border of the clear cells. In the centre of the embryo is a segmentation cavity (c.s.) which lies between the granular and the clear cells, but is mainly bounded by the vaulted inner surface of the latter. This stage is known as theamphiblastulastage. During the later periods of the amphiblastula stage a cavity appears in the granular cells dividing them into two layers. After the larva has for some time enjoyed a free existence, a remarkable series of changes take place, which result in the invagination of the half of it formed of the clear cells, and form a prelude to the permanent attachment of the larva. The entire process of invagination is completed in about half an hour. The whole embryo first becomes flattened, but especially the ciliated half, which gradually becomes less prominent (fig. 65B); and still later the cells composing it undergo a true process of invagination. As a result of this invagination the segmentation cavity is obliterated, and the larva assumes a compressed plano-convex form, with a central gastrula cavity, and a blastopore in the middle of the flattened surface. The two layers of the gastrula may now be spoken of as epiblast and hypoblast. The blastopore becomes gradually narrowed by the growth over it of the outer row of granular cells. When it has become very small the attachment of the larva takes place by the flat surface where the blastopore is situated. It is effected by protoplasmic processes of the outer ring of epiblast cells, which, together with the otherepiblast cells, now become amœboid. They become at the same time clearer and permit a view of the interior of the gastrula. Between the epiblast cells and the hypoblast cells which line the gastrula cavity there arises a hyaline structureless layer, which is more closely attached to the epiblast than to the hypoblast, and is probably derived from the former. A view of the gastrula stage after the larva has become fixed is given infig. 66.
Fixed Gastrula stage of Sycandra raphanusFig. 66. Fixed Gastrula stage of Sycandra raphanus.(Copied from Schulze.)The figure shews the amœboid epiblast cells (ec.) derived from the granular cells of the earlier stage, and the columnar hypoblast cells, lining the gastrula cavity, derived from the ciliated cells of the earlier stage. The larva is fixed by the amœboid cells on the side on which the blastopore is situated.
Fig. 66. Fixed Gastrula stage of Sycandra raphanus.(Copied from Schulze.)
The figure shews the amœboid epiblast cells (ec.) derived from the granular cells of the earlier stage, and the columnar hypoblast cells, lining the gastrula cavity, derived from the ciliated cells of the earlier stage. The larva is fixed by the amœboid cells on the side on which the blastopore is situated.
There would seem according to Metschnikoff’s observations (No.134) to be a number of mesoblast cells interposed between the two primary layers, which he derives from the inner part of the mass of granular cells.
After invagination the cilia of the hypoblast cells can no longer be seen, and are probably absorbed; and their disappearance is nearly coincident with the complete obliteration of the blastopore, an event which takes place shortly after the attachment of the larva.
Not long after the closure of the blastopore, calcareous spicules make their appearance in the larva as delicate unbranched rods pointed at both extremities. They appear to be formed on the mesoblast cells situated between the epiblast and hypoblast[67]. The larva when once fixed rapidly grows in length and assumes a cylindrical form (fig. 67A). The sides of the cylinder are beset with calcareous spicules which project beyond the surface, and, in addition to the unbranched forms, spicules are developed with three and four rays as well as some with a blunt extremity and serrated edge. The extremity of the cylinder opposite the attached surface is flattened, and, though surrounded by a ring of four-rayed spicules, is itself free from them. At this extremity a small perforation is formed leading into the gastric cavity, which rapidly increases in size and forms an exhalent osculum (os.). A series of inhalent apertures is also formed at the sides of the cylinder. The relative times of appearance of the single osculum and the smaller apertures are not constant for the different larvæ. On the central gastrula cavity of the sponge becoming placed in communication with the external water, the hypoblast cells lining it become ciliatedafresh (fig. 67B,en.) and develop the peculiar collar characteristic of the hypoblast cells of the Spongida (videfig. 64,hy.). When this stage of development is reached we have a fully formed sponge of the type made known by Haeckel as Olynthus.
The young of Sycandra raphanusFig. 67. The young of Sycandra raphanus shortly after the development of the Spicula.(Copied from Schulze.)A. View from the side.B. View from the free extremity.os.osculum;ec.epiblast;en.hypoblast composed of ciliated cells. The terminal osculum and lateral pores are represented as oval white spaces.
Fig. 67. The young of Sycandra raphanus shortly after the development of the Spicula.(Copied from Schulze.)
A. View from the side.B. View from the free extremity.
os.osculum;ec.epiblast;en.hypoblast composed of ciliated cells. The terminal osculum and lateral pores are represented as oval white spaces.
When young examples of Sycandra come in contact shortly after their attachment they appear to fuse together temporarily or else permanently. In the latter case colonies are produced by their fusion.
Amongst other calcareous sponges the larva ofAscandra contorta(HaeckelNo.126, BarroisNo.122) presents the typical amphiblastula stage, and so probably does that ofAscandra Lieberkühnii(KellerNo.128). InLeucandra aspera(KellerNo.128, MetschnikoffNo.134) the larva passes through an amphiblastula stage, but the characters of the cells of the two halves of the larva do not differ to nearly the same extent as in Sycandra.
Although the majority of calcareous sponges appear to agree in theirmode of development with Sycandra, nevertheless the concordant researches of O. Schmidt (No.138) and Metschnikoff (No.134) have shewn that this is not true for the genus Ascetta (As. primordialis,clathrusandblanca).
The larvæ of these forms are very differently constituted to those of Sycandra. They have an oval form and are composed of a single row of ciliated columnar cells: their two extremities only differ in the cells at one extremity being longer than those at the other. Especially at the pole where the shorter cells are situated (Schmidt) a metamorphosis of the cells takes place. One after the other they lose their cilia, become granular, and pass into the interior of the vesicle. Here they become differentiated into two classes (Metschnikoff); one of larger and more granular cells, and the other of smaller cells with clearer protoplasm. Cells of the former class are mainly found at one of the poles. When the larva becomes free the cells in the interior of the vesicle increase in number and nearly fill up its central cavity. After a short free existence the larva becomes fixed, and the epiblast cells lose their cilia and become flattened. At a later period the large granular cells assume a radiate arrangement round a central cavity and become clearly marked out as the hypoblast cells. The smaller cells become placed between the epiblast and hypoblast and constitute the mesoblast.
Myxospongiæ.In this group Halisarca has been investigated by Carter (No.123), Barrois (No.122), Schulze (No.141) and Metschnikoff (No.134). The ova develop in the mesoblast, and when ripe occupy special chambers lined by a layer of epithelial cells. Schulze has found the spermatozoa of this genus of sponge and has been able to shew that the sexes may be distinct, though many species of Halisarca are hermaphrodite.
The segmentation is, roughly speaking, regular, and a segmentation cavity is early formed, which is never, as in Calcispongiæ, open at the poles. When the larva leaves the parent it is an oval vesicle formed of a single layer of columnar ciliated cells. Slight differences may be observed between the two extremities of the larvæ of most species. One of these—the hinder extremity—is directed backwards in swimming.
The further history of the larva has been investigated by Metschnikoff. He has found that the interior of the vesicle becomes gradually filled with mesoblast cells of a peculiar type, called by him rosette-cells, which are probably derived from the walls of the vesicle.
When the metamorphosis commences, the larva assumes a flattened form, and cells of a new type,viz.normal amœboidcells, grow in amongst the rosette cells. The new cells are also derived from the epiblast. The larvæ appear to fix themselves by the hinder extremity. The cilia gradually disappear, and the epiblast cells flatten out and form a kind of cuticle. For some time the larva remains in the two-layered condition, but gradually canals (? ciliated chambers) lined by hypoblast cells become formed. They appear as closed spaces with walls of ciliated cells derived from the amœboid cells, and the different parts of the system of chambers are established independently. InH. ponticathe ciliated chambers are formed before the attachment of the larva. The development was not followed up to the formation of the pores placing the canal system in communication with the exterior.
The young sponges at a somewhat later stage have been studied by Schulze and Barrois. They are formed of an external layer of flattened cells, not clearly ciliated as in the adult, within which are a normal mesoblastic tissue, and several spherical chambers lined by ciliated cells exactly like the ciliated chambers of the full-grown sponge. Irregular invaginations of the epiblast give to the young sponge a honeycombed structure. The ciliated chambers in the youngest condition of the sponge are closed; but in slightly older examples they come into communication with the passages lined by hypoblast, and so indirectly with the external medium.
Ceratospongiæ.Amongst the true Ceratospongiæ the embryos of two of the Aplysinidæ, and of Spongelia and Euspongia have been to some extent worked out by Barrois and Schulze. The form worked out by Barrois is called by himVerongia rosea. The segmentation is nearly regular, but from the first the segments may be divided according to their constitution into two categories. At the close of segmentation the embryo is oval and covered by a single layer of columnar ciliated cells; these cells may however be divided into two categories, corresponding with those observable during the segmentation. A certain number are coloured red and form a definite circular mass at one pole, while the remainder, which constitute the major part of the embryo, have a pale yellowish colour. Those at the red pole lose their cilia in the free larva, but around the area formed by them is a special ring of long cilia. The chief peculiarity of the embryo (made known by Schulze) consists in the fact that the layer of cells which covers the embryo does not, as in other sponge embryos, simply enclose a space, but the interior of the embryo is formed of a mass of stellate cells like the normal mesoblast of full-grown sponges.
This feature is also characteristic of the embryos of Spongelia and Euspongia.
The embryo of the Gummineæ (Gummina mimosa) has been investigated by Barrois (No.122), and has been shewn closely to resemble the typical larvæ of calcareous sponges; one-half being formed ofelongated ciliated cellsand the other of rounded granular ones.
Silicispongiæ.The development of marine silicious sponges is but very imperfectly understood. The larvæ of various forms—Reniera (Isodyctia), Esperia (Desmacidon), Raspailia, Halichondria, Tethya—have been described. Barrois has shewn that the egg segments regularly and that in the earlier stages a segmentation cavity is present. In the later stages the embryo appears to become solid. Externally there is a layer of ciliated cells, and within a mass of granular matter in which the separate cells cannot be made out. The granular matter projects at one pole, and forms a prominence possibly equivalent to the granular cells of Sycandra. In some forms,e.g.Reniera, the edge of the unciliated granular prominence may be surrounded by a row of long cilia. In later stages the granular material may project at both poles or even at other points. One remarkable feature in the development of the Silicispongiæ is the appearance of spicula between the ciliated cells and the central mass, while the larva is still free.
Professor Schulze has informed me that these spicula are developed in mesoblast cells; while the horny fibres of the sponge are developed as cuticular products of special mesoblast cells (spongioblasts).
The attachment and accompanying metamorphosis are so diversely described that no satisfactory account can be given of them. The general statements are in favour of the attachment taking place by the posterior extremity where the granular matter projects.
Carter especially gives a very precise account, with figures, of the attachment of the larva in this way. He also figures the appearance of an osculum at the opposite pole[68].
A very elaborate account of the development of Spongilla has been published in Russian by Ganin, of which a German abstract has also appeared (No.124).
The ovum undergoes a regular segmentation and becomes a solid ova morula. An epiblast of smaller cells is early differentiated, and in the interior of the inner cells an archenteron becomes subsequently formed. The inner cells next become divided into an hypoblastic layer lining thearchenteron, and a mesoblastic layer between this and the now ciliated epiblast. At the narrow hinder end of the embryo the mesoblast becomes thickened, and largely obliterates the archenteron. In this part of the mesoblast silicious spicula are formed. The larva becomes attached by its hinder extremity, and in the course of this process flattens itself out to a disc-like form. From the nearly obliterated archenteric cavity outgrowths take place which give rise to the ciliated chambers. These are not placed directly in communication with the exterior, but open, if I understand Ganin rightly, into a space in the mesoblast, which subsequently acquires an exterior communication—the primitive osculum. The subsequent pores and oscula are also formed as openings leading into the mesoblastic cavity, which communicates in its turn with the ciliated chambers.
It appears that in the present unsatisfactory state of our knowledge the larvæ of the Porifera may be divided into two groups:viz.(1) those which have the form of a blastosphere or else of a solid morula; (2) those which have the amphiblastula form.
In the former type the mesoblast and hypoblast are formed either from cells budded off from the outer cells of the blastosphere or from the solid inner mass of cells; while the outer ciliated cells become the epiblast. This type of larva, which is found in the majority of sponges, is very similar in its general characters and development to many Cœlenterate planulæ.
The second type of larva is very peculiar, and though in its fully developed form it is confined to the Calcispongiæ, where it is the usual form, a larval type with the same characters is perhaps to be found in other sponges,e.g.amongst the Gumminæ, and amongst the Silicispongiæ where one-half of the embryo is without cilia, though in the case of the Silicispongiæ the cells of the ciliated part of the embryo correspond to the granular cells of the larva of Sycandra.
The later stages in the development of the larvæ of the Porifera are not similar to anything we know of in other groups.
It might perhaps be possible to regard sponges as degraded descendants of some Actinozoon type such as Alcyonium, with branched prolongations of the gastric cavity, but there does not appear to me to be sufficient evidence for doing so at present. I should rather prefer to regard them as an independent stock of the Metazoa.
In this connection the amphiblastula larva presents some points of interest. Does this larva retain the characters of an ancestral type of the Spongida, and if so, what does its form mean? It is, of course, possible thatit has no ancestral meaning but has been secondarily acquired; but, assuming that this is not the case, it appears to me that the characters of the larva may be plausibly explained by regarding it as a transitional form between the Protozoa and Metazoa. According to this view the larva is to be considered as a colony of Protozoa, one-half of the individuals of which have become differentiated into nutritive forms, and the other half into locomotor and respiratory forms. The granular amœboid cells represent the nutritive forms, and the ciliated cells represent the locomotor and respiratory forms. That the passage from the Protozoa to the Metazoa may have been effected by such a differentiation is not improbable ona priorigrounds.
While the above view seems fairly satisfactory for the free-swimming stage of the larval sponge, there arises in the subsequent development a difficulty which appears at first sight fatal to it. This difficulty is the invagination of the ciliated cells instead of the granular ones. If the granular cells represent the nutritive individuals of the colony, they, and not the ciliated cells, ought most certainly to give rise to the lining of the gastrula cavity, according to the generally accepted views of the morphology of the Spongida. The suggestion which I would venture to put forward in explanation of this paradox involves a completely new view of the nature and functions of the germinal layers of adult Spongida.
It is as follows:—When the free-swimming ancestor of the Spongida became fixed, the ciliated cells by which its movements used to be effected must have to a great extent become functionless. At the same time the amœboid nutritive cells would need to expose as large a surface as possible. In these two considerations there may, perhaps, be found a sufficient explanation of the invagination of the ciliated cells, and the growth of the amœboid cells over them. Though respiration was, no doubt, mainly effected by the ciliated cells, it is improbable that it was completely localized in them, but they were enabled to continue performing this function through the formation of an osculum and pores. The collared cells which line the ciliated chambers, or in some cases the radial tubes, are undoubtedly derived from the invaginated cells, and, if there is any truth in the above suggestion, the collared cells in the adult sponge must be mainly respiratory and not digestive in function, while the epiblastic cells, which in most cases line the inhalent passages through its substance[69], ought to be employed to absorb nutriment. The recent researches of Metschnikoff (No.134) on this head shew that the nutriment is largely carried into the mesoblast cells, which in Sycandra appear to be derivedfrom the granular cells, and also that it is taken up by the cells which line the passages, though not by the superficial epiblast cells. Whether the collared cells generally absorb nutriment is not clear from his statements: buthe finds that they do not do so in Silicispongiæ.
Professor Schulze has informed me by letter that he finds the collared cells to be respiratory in function, while the cells derived from the granular cells in Sycandra are nutritive. Carter[70], on the contrary, from his observations on Spongilla, has fully satisfied himself that the food is absorbed by the cells lining the ciliated chambers.
If it is eventually proved by further experiments on the nutrition of sponges, that digestion is mainly carried on by the general cells lining the passages and the mesoblast cells, and not for the most part by the ciliated cells, it is clear that the epiblast, mesoblast and hypoblast of sponges will not correspond with the similarly named layers in the Cœlenterata and other Metazoa. The invaginated hypoblast will be the respiratory layer and the epiblast and mesoblast the digestive and sensory layers; the sensory function being probably mainly localized in the epithelium on the surface, and the digestive one in the epithelium lining the passages and in the mesoblast. Such a fundamental difference in the primary function of the germinal layers between the Spongida and the other Metazoa, would necessarily involve the creation of a special division of the Metazoa for the reception of the former group.
Bibliography.
(122)C. Barrois.“Embryologie de quelques éponges de la Manche.”Annales des Sc. Nat. Zool.,VI.ser., Vol.III.1876.(123)Carter. “Development of the Marine Sponges.”Annals and Mag. of Nat. Hist.,4thseries,Vol.XIV.1874.(124)Ganin[71].“Zur Entwicklung d. Spongilla fluviatilis.”Zoologischer Anzeiger.Vol.I.No.9, 1878.(125)Robert Grant. “Observations and Experiments on the Structure and Functions of the Sponge.”Edinburgh Phil. J.,Vol.XIII.andXIV., 1825, 1826.(126)E. Haeckel.Die Kalkschwämme, 1872.(127)E. Haeckel.Studien zur Gastræa-Theorie.Jena, 1877.(128)C. Keller.Untersuchungen über Anatomie und Entwicklungsgeschichte einiger Spongien.Basel, 1876.(129)C. Keller.“Studien üb. Organisation u. Entwick. d. Chalineen.”Zeit. f. wiss. Zool.,Bd.XXVIII.1879.(130)Lieberkühn.“Beitr. z. Entwick. d. Spongillen.”Müller’sArchiv, 1856.(131)Lieberkühn.“Neue Beiträge zur Anatomie der Spongien.”Müller’sArchiv, 1859.(132)El. Metschnikoff.“Zur Entwicklungsgeschichte der Kalkschwämme.”Zeit. f. wiss. Zool.,Bd.XXIV.1874.(133)El. Metschnikoff.“Beiträge zur Morphologie der Spongien.”Zeit. f. wiss. Zool.,Bd.XXVII.1876.(134)El. Metschnikoff.“Spongeologische Studien.”Zeit. f. wiss. Zool.,Bd.XXXII.1879.(135)Miklucho-Maklay.“Beiträge zur Kenntniss der Spongien.”Jenaische Zeitschrift,Bd.IV.1868.(136)O. Schmidt.“Zur Orientirung über die Entwicklung der Schwämme.”Zeit. f. wiss. Zool.,Bd.XXV.1875.(137)O. Schmidt.“Nochmals die Gastrula der Kalkschwämme.”Archiv für mikrosk. Anat.,Bd.XII.1876.(138)O. Schmidt.“Das Larvenstadium von Ascetta primordialis und Asc. clathrus.”Archiv für mikrosk. Anatomie,Bd.XIV.1877.(139)F. E. Schulze.“Ueber den Bau und die Entwicklung von Sycandra raphanus.”Zeit. f. wiss. Zool.,Bd.XXV.1875.(140)F. E. Schulze.“Zur Entwicklungsgeschichte von Sycandra.”Zeit. f. wiss. Zool.,Bd.XXVII.1876.(141)F. E. Schulze.“Untersuchung üb. d. Bau, etc. Die Gattung Halisarca.”Zeit. f. wiss. Zool.,Bd.XXVIII.1877.(142)F. E. Schulze.“Untersuchungen üb. d. Bau, etc. Die Metamorphose von Sycandra raphanus.”Zeit. f. wiss. Zool.,Bd.XXXI.1878.(143)F. E. Schulze.“Untersuchungen ü. d. Bau, etc. Die Familie Aplysinidæ.”Zeit. f. wiss. Zool.,Bd.XXX.1878.(144)F. E. Schulze.“Untersuchungen ü. d. Bau, etc. Die Gattung Spongelia.”Zeit. f. wiss. Zool.,Bd.XXXII.1878.
[67]Metschnikoff was the first to give this account of the development of the spicules in Sycandra, but Prof. Schulze has informed me by letter that he has arrived at the same result.[68]Keller (No.129) has recently given an account of the development of Halichondria (Chalinula) fertilis. He finds that there is an irregular segmentation, followed by a partial epibolic invagination, the inner mass of cells remaining exposed at one pole and forming there a prominence, equivalent to the granular prominence in the larvæ of other Silicispongiæ. The free-swimming larva resembles the larva of other Silicispongiæ in the possession of spicula, etc., and after becoming laterally compressed attaches itself by one of the flattened sides. A central cavity is formed in the interior with ciliated chambers opening into it, and is subsequently placed in communication with the exterior by the formation of an aperture which constitutes the osculum.[69]That the greater part of the flat cells which line the passages of most Sponges are really derived from epiblastic invaginations appears to me to be proved by Schulze’s and Barrois’ observations on the young fixed stages of Halisarca. Schulze’s (No.140) observations have however proved that the flat cells lining the axial gastric chamber of Sycandra are hypoblastic in origin, and the observations of Keller (No.129) and Ganin (No.124) have led to the same result for the flat epithelium lining part of the passages of the Silicispongiæ.[70]“On the Nutritive and Reproductive Processes of Sponges.”Ann. and Mag. of Nat. Hist.,Vol.IV.Ser.V.1879.[71]There is a Russian paper by the same author, containing a full account, with clear illustrations, of his observations.
[67]Metschnikoff was the first to give this account of the development of the spicules in Sycandra, but Prof. Schulze has informed me by letter that he has arrived at the same result.
[68]Keller (No.129) has recently given an account of the development of Halichondria (Chalinula) fertilis. He finds that there is an irregular segmentation, followed by a partial epibolic invagination, the inner mass of cells remaining exposed at one pole and forming there a prominence, equivalent to the granular prominence in the larvæ of other Silicispongiæ. The free-swimming larva resembles the larva of other Silicispongiæ in the possession of spicula, etc., and after becoming laterally compressed attaches itself by one of the flattened sides. A central cavity is formed in the interior with ciliated chambers opening into it, and is subsequently placed in communication with the exterior by the formation of an aperture which constitutes the osculum.
[69]That the greater part of the flat cells which line the passages of most Sponges are really derived from epiblastic invaginations appears to me to be proved by Schulze’s and Barrois’ observations on the young fixed stages of Halisarca. Schulze’s (No.140) observations have however proved that the flat cells lining the axial gastric chamber of Sycandra are hypoblastic in origin, and the observations of Keller (No.129) and Ganin (No.124) have led to the same result for the flat epithelium lining part of the passages of the Silicispongiæ.
[70]“On the Nutritive and Reproductive Processes of Sponges.”Ann. and Mag. of Nat. Hist.,Vol.IV.Ser.V.1879.
[71]There is a Russian paper by the same author, containing a full account, with clear illustrations, of his observations.
Hydroidea. The most typical mode of development of the Hydroidea is that in which the segmentation leads directly to the formation of a free ciliated two-layered larva, known since Dalyell’s observations as a planula. The planula is characteristic of almost all the Hydromedusæ with fixed hydrosomes including the Hydrocoralla (Stylasteridæ and Millepora), the most important exceptions being the genus Tubularia and one or two other genera, and the fresh-water Hydra.
In a typical Sertularian the segmentation is approximately regular[73]and ends according to the usual accounts in the formation of a solid spherical mass of cells. A process of delamination now takes place, which leads to the formation of a superficial layer of cubical or pyramidal cells, enclosing a central solid mass of more or less irregularly arranged cells.
The embryo, in the cases in which it is still contained within the sporosack, now begins to exhibit slight changes of form, andone extremity of it begins to elongate. It soon becomes free, and rapidly assumes an elongated cylindrical form, while a coating of cilia, by means of which it moves sluggishly about, appears on its outer surface. A central cavity appears in the interior, and the inner cells form themselves into a definite hypoblast. The larva has now become a planula, and consists of a closed sack with double walls. It continues for some few days to move about, but eventually drops its cilia, and becomes dilated at one extremity, by which it then becomes attached. The base of attachment becomes gradually enlarged so as to form a disc, which spreads out and is frequently divided by fissures into radiating lobes. The free extremity becomes enlarged to form the eventual calyx.
Over the whole exterior a delicate pellicle—the future perisarc—now becomes secreted. Round the edge of the anterior enlargement a row of tentacles makes its appearance. These, in the embryos of the Tubularian genera, lie some little way behind the apex of the body. After a certain time the perisarc, which has hitherto been continuous, becomes ruptured in the region of the calyx, and the tentacles become quite free. At about the same period a mouth is formed at the oral apex.
Three larva stages of Eucope polystylaFig. 68. Three larva stages of Eucope polystyla.(After Kowalevsky.)A. Blastosphere stage with hypoblast spheres becoming budded off 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. 68. Three larva stages of Eucope polystyla.(After Kowalevsky.)
A. Blastosphere stage with hypoblast spheres becoming budded off into the central cavity.B. Planula stage with solid hypoblast.C. Planula stage with a gastric cavity.
ep.epiblast;hy.hypoblast;al.gastric cavity.
The development of Eucope polystyla (fig. 68), one of the Campanularidæ, deviates according to Kowalevsky (No.147) in somewhat important points from the usual type. The whole development takes place after the deposition of the ovum. The segmentation results in the formation of a single-walled blastosphere with a large central cavity (fig. 68A). This cavity, somewhat as in Ascetta, becomes filled up with a not clearly (?) cellular material derived from the walls of the blastosphere, which must be regarded as the hypoblast (fig. 68B). The larva elongates and becomes ciliated, and the epiblast at its two extremities becomes thickened, and is stated by Kowalevsky also to become divided into two layers. The alimentary cavity appears as a slit in the middle of the hypoblast (fig. 68C). The cilia after a time disappear, and the larva then becomes fixed by one extremity. It flattens itself out into a disc-like form, becomes divided into four lobes, and covered by a cuticle (perisarc). From the disc the stalk grows out which dilates at its free extremity into the calyx.
Section of T. mesembryanthemumFig. 69. Longitudinal section through a larva of Tubularia mesembryanthemum while still in the gonophore.The lower end is the oral one.ep.epiblast;hy.hypoblast of tentacle;en.enteric cavity.
Fig. 69. Longitudinal section through a larva of Tubularia mesembryanthemum while still in the gonophore.The lower end is the oral one.
ep.epiblast;hy.hypoblast of tentacle;en.enteric cavity.
In both the groups (Tubularia and Hydra) which are exceptional in not having a ciliated planula stage, its absence may be put down to an abbreviation of the development, and in fact a two-layered quiescent stage, through which the embryo passes, may be regarded as representing the planula stage.
The development of Tubularia, which has been described in detail by Ciamician, takes place in the gonophore[74]. The segmentation is irregular and leads to the formation of an epibolic gastrula, four large central cells constituting the hypoblast[75]. The larva now elongates, and grows out laterally into two processes which constitute the first pair of tentacles. At this stage it closely resembles the larvæ of some Medusæ. Additional tentacles are soon formed; and a central cavity appears in the hypoblast, the cells of which have in the meantime become more numerous (fig. 69). The tentacles are directed towardsthe aboral side, which is considerably more prominent than the oral one. They contain a hypoblastic axis. The aboral end continues to grow and the tentacles gradually assume a horizontal position. A constriction now appears, dividing the larva into an aboral portion which will eventually form the stalk, and an oral portion. At the apex of the latter a row of short tentacles—the future oral tentacles—now appears. The larva has at this stage the form known as Actinula. In this condition it becomes hatched, and shortly afterwards it becomes fixed by the aboral end and grows into a colony.
The development of Myriothela (Allman,No.150) takes place on the Tubularian type. The ovum invested by a delicate capsule becomes freed by the rupture of the gonophore, and is then taken up by the remarkable claspers characteristic of the genus. In the claspers it becomes fecundated and undergoes its further development. After segmentation a gastric cavity is formed, and provisional tentacles arise as a series of conical involutions which subsequently become evoluted. Permanent tentacles are formed as conical papillæ on a truncated oral process. After hatching it has a few days’ free existence, and then becomes attached, and loses its provisional tentacles.
Although Hydra itself constitutes the simplest type of Hydrozoon, its development, which has been fully investigated by Kleinenberg (No.161), is in some respects a little exceptional. The segmentation is regular, but a segmentation cavity is not formed. The peripheral layer of cells gradually becomes converted into a chitinous membrane, which is perhaps homologous with the perisarc of marine forms. Between the membrane and the germ a second pellicle makes its appearance. The above changes require about four days for their completion, but there next sets in a period of relative quiescence which lasts for some 6‑8 weeks. During this period the remaining development is completed. The cells of the germ first fuse together. In the interior of the protoplasm a clear excentric space arises, which gradually extends itself and forms the rudiment of the gastric cavity. The outer shell in the meantime becomes less firm, and is finally burst and thrown off, owing to the expansion of the embryo within.
The outermost layer of the protoplasm becomes, relatively to the inner layer, clear and transparent, and there thus arises an indication of a division of the walls of the archenteric cavity into two zones, or layers. These layers, which form the epiblast and hypoblast, are definitely established on the appearance of cells with contractile tails[76]in the clear outer zone, between which the interstitial epiblast cells subsequently arise.
The embryo, still forming a closed double-walled sack, elongates itself, and at one pole its wall becomes very thin. And at this point a rupture takes place which gives rise to the mouth. Simultaneously with the mouth the tentacles become formed as hollow processes, according to Mereschkowsky two being formed first and subsequently the others in pairs. Very shortlyafterwards the hitherto uniform hypoblast becomes divided up into distinct cells. The thin inner pellicle which persists after the rupture of the outer membrane becomes in the meantime absorbed. With these changes the embryo practically acquires the characters of the adult.
Trachymedusæ.Amongst the Trachymedusæ, which as has now been satisfactorily established develop directly without alternations of generations, the embryology of species both of the Geryonidæ and the Æginidæ has been studied.
In all the types so far investigated the hypoblast is formed by delamination, and there is a more or less well-marked planula stage.