Chap. III.] 



SPONGES. 



35 



once for a character which sharply distinguishes from 



all other animals. It happens to many Gastrulse 



that, their blastopore closing up, they develop an 



investment of cilia on their epi- 



blast, and swim about for a 



time freely in the water; but 



these cilia are confined to the 



outer surface. In the sponges 



it is otherwise, the ciliated cells 



early become internal to the non- 

 ciliated, and some are retained 



throughout life in the so-called 



" ciliated chambers." When we 



come to examine into the activity 

 of a living sponge we find no 

 advance on that of a Protozoon, 

 save so far as the division of 



labour is here first clearly seen ; 



we find, that is, that the multi- 



celluiai\ organism feeds, grows, 



respires, reproduces itself, and 



dies ; and we find, too, that, like 



many Protozoa, it forms for 



itself firm supports in the way 



of a skeleton, but we find no 



cells that are specially sensory, 



and none that are obviously 



muscular ; there is the general 



irritability and contractility 



which living protoplasm always exhibits, but there 



are no special organs for either function. 



The Porifera, or sponges, fall into the following 

 divisions : 



1. Ittyxospoiigiae, in which there is no hard 

 skeleton ; e.g. Halisarca. 



2. Calcispoiigiae, in which a support for the 

 body is furnished by calcareous spicules ; e.g. Ascon, 



Fig. 7. Calcareous 

 Sponge. AKcetta pn'mor- 

 ctoa7f*. (After Haeckel, 

 x SOdiams.) 



