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 the archenteron, 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.