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. 67] A). 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 ciliated afresh ([fig. 67] B, en.) and develop the peculiar collar characteristic of the hypoblast cells of the Spongida (vide [fig. 64], hy.). When this stage of development is reached we have a fully formed sponge of the type made known by Haeckel as Olynthus.
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 of Ascandra contorta (Haeckel No. [126], Barrois No. [122]) presents the typical amphiblastula stage, and so probably does that of Ascandra Lieberkühnii (Keller No. [128]). In Leucandra aspera (Keller No. [128], Metschnikoff No. [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 their mode 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, clathrus and blanca).