220 



(XELENTEBA.TA. 



After the completion of the tolerably regular segmentation (fig. 

 1G6, a c), Sycon (Sycandra') raphanus passes through a blastosphere 

 stage, dining which the greater half of the ovum consists of clear 

 cylindrical cells, and the smaller half at the still open pole of large 

 dark granular cells (tig. 166, d). The cylindrical cells of the larger 

 half develop cilia, and the embyro passes out of the body cavity and 

 becomes a free-swimming larva, which attaches itself and alters its 

 shape in such a manner that the dark cells grow over the ciliated 

 portion of the globe, which is meanwhile invaginating. The ecto- 

 derm and mesoderni are derived from the dark granular cells, arid 



the ciliated cells 

 give rise to the 

 entoderna of the 

 gastric cavity. 

 Later on the body 

 of the sponge be- 

 comes cylindrical, 

 the osculum makes 

 its appearance, and 

 calcareous needles 

 appear in the wall, 

 which becomes 

 pierced by pores 

 (fig. 167). 



With the excep- 

 tion of Spongilla, 

 the sponges are 



FIG. 1G7. Young Sycon (after Fr. E. Scliulze). O, Osculum 

 or exlialent aperture ; P, pores of the wall. 



marine, and are 



met with under 

 very different con- 

 ditions, and covering a wide area of distribution. The horny 

 sponges live in shallow seas, as also the Myxospongice and Cficdinece, 

 or siliciceratous Sponges ; while the Hexactinellidce inhabit very 

 considerable depths. Petrified remains of sponges are also found 

 preserved in various formations, for instance in the chalk ; and 

 these remains differ much from the greater number of those 

 living. On the other hand, the hyaline sponges of the deep sea 

 agree so fully with the ancient forms that they seem to be the 

 direct descendants of the latter. Finally, many of the principal 

 groups extend back into the palaeozoic age, in which LithistidcE and 

 Hexactinellidce especially are met with in the most ancient Silurian 



