8 



SPONGES 



CHOANOFLAGELLATA 



That the objects of everyday use known as sponges are the 

 skeletons of marine animals is tolerably common knowledge ; 

 but what may be the nature of the soft parts of such animals 

 is less well known. A hving sponge, however, is not only a very 

 remarkable, but even a fairly highly organised creature. Its 

 nearest relations, outside its own sub-kingdom, are to be found 

 among the flagellate Protozoa. Certain of these, known as the 

 Choanoflagellata, with a shape of body not unUke that of Poly- 

 toma, but having Hke Trypanosoma a single fiagellum, are fastened 

 by a stalk at one end, and have at the other end the fiagellum, 

 and around it a delicate collar of protoplasm. Fig. 49, a shows 

 one of these organisms. The Choanoflagellata often secrete a 

 cup or case which houses and protects the body (Fig. 49, h) and 

 sometimes the cups form a kind of colony, being joined together 

 to give a shape Hke that of a filamentous alga (Fig. 49, c) ; but in 

 one of them, known as Proterospongia (Fig. 50), the substance 

 secreted takes the form of a mass of jelly, in which a number of 

 separate individuals are lodged. Some stand on the outside, half- 

 imbedded, with the collared end projecting ; others pass into the 

 jelly and there become amoeboid, and some of these divide into 

 minute spores, which may be gametes. The simplest sponge is a 

 multicellular organism, the elements of whose body suggest a 

 rearrangement of those of the colony of Proterospongia. 



A SIMPLE SPONGE 



This simple sponge is a little creature, known as the Olynthus, 

 which is found only as a fleeting stage in the development of 

 a few members of the group ; but the bodies of all may be regarded 

 as ideally derived from it, even though it may not appear as a 

 stage in their life-history. It is a hollow vase, perforated by many 

 pores, and having at the summit a single large opening, the 

 osculum. Through the pores water constantly enters it, to pass out 



through the osculum. Herein it and its kind differ from all other 



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