CHAPTER XXVI 



SPONGES 



Vacuole 



THE PHYLUM PORIFERA 



In the ocean are found many animals which would not be recognized 

 as such by the ordinary observer, since they have neither power of move- 

 ment nor power of locomotion, and since they form inert masses attached 

 to various solid objects including the shells of other living animals. In 

 many cases these are sponges, though some ascidians (Sec. 338) would 

 fit the description. Sponges were long supposed to be plants and their 

 animal nature was not fully established until about 1857, since which 

 time they have been variously classified 

 in the animal kingdom. 



148. Relationship of Sponges. — 

 In many respects sponges are like 

 colonial protozoans. For instance, 

 they possess collar cells (Fig. 49) 

 which are similar to the collared cells 

 of the colonial flagellate protozoan, 

 Proterospongia (Fig. 27A).' On this 

 account the sponges were for a time 

 classified as colonial flagellate Pro- 

 tozoa. They differ from them, how- 

 ever, in the fact that the body is 

 penetrated by a system of canals, whereas in colonial Protozoa the cells 

 are upon the surface of the mass formed by the colony. They also 

 differ in the fact that there are a number of different types of nonre- 

 productive, or somatic, cells which perform different functions. In this 

 respect sponges resemble higher animals. They have, therefore, been 

 considered for some time to be Metazoa. 



Sponges differ fundamentally from other Metazoa by not having any 

 digestive cavity, which is present in some form in all higher animals 

 except where lost from degeneration. Instead, digestion is always 

 intracellular, or wathin the cells, as in Protozoa. Neither do they have 

 body layers corresponding exactly to those in other Metazoa, in which 

 ectoderm, mesoderm, and entoderm retain from the beginning the same 

 relative position in the body. In the sponges the layer which appears in 

 the embryo as an ectodermal layer comes in the course of development 

 to line central cavities and to have the function of circulating water 



127 



Nucleus 



Fig. 49. — A number of collar cells, or 

 choanocytes, from one of the flagellated 

 chambers of a fresh-water sponge, Spon- 

 gilla lacustris (Linnaeus). {From Borra- 

 dailc and Potts, " The Invertebrata," after 

 Vosmaer, by the courtesy of The Macmillan 

 Company.) 



