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- 

 cellular 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. Myxospongise, in which there is 110 hard 

 skeleton ; e.g. Halisarca. 



2. Calcispongiae, in which a support for the 

 body is furnished by calcareous spicules ; e.g. A scon, 



Fig. 7. Calcareous 

 Sponge. Ascetta primor- 

 <7 in 7 is. (After Haeckel, 

 x SOdiarus.) 



