388 GENERAL ZOOLOGY 



Separate sexes occur in some species of sponges; others are 

 hermaphroditic. The fertilized egg develops into a two-layered, 

 ciliated, motile larva. The gastral cavity and its derivatives 

 develop from the outer layer of the larva; and the dermal layer 

 and mesenchyme come from the inner layer of the larva. Thus 

 there is in sponges what is called an inversion of the germ layers, 

 since in other metazoa the outer layer of the gastrula gives rise to 

 ectoderm and the inner layer to endoderm. At metamorphosis 

 the ciliated sponge larva becomes attached to the substratum 

 and is transformed into a sponge. 



Sponges lack the usual organ systems found in higher Metazoa. 

 There is no nervous system. The flagella of the collar cells beat 

 independently and not in unison, as do the cilia of ciliated epi- 

 thelia of other metazoans. The currents created in the water 

 by the flagella bring in food and oxygen and remove waste. 

 There is no other circulatory or excretory system. The myocytes 

 located about the pores and the osculum represent a very primi- 

 tive form of contractile tissue which combines the properties of 

 nerve and muscle, since it responds normally to direct stimula- 

 tion. In keeping with their low degree of organization, sponges 

 display a remarkable power of regeneration. If sponges, of 

 certain species, are crushed to a pulp and pressed through fine 

 bolting cloth, the dissociated cells collect in small masses, some 

 of which eventually develop into normal sponges. 

 CLASS I. CALCAREA. Marine sponges, small in size, 



tubular in shape, solitary or colonial, with calcareous spicules. 

 Example : Grantia canadensis, a common species of the Sycon 



type. 

 CLASS II. NONCALCAREA. Marine and fresh-water 



forms with siliceous spicules or spongin, or both, or neither. 

 Examples: Euplectella aspergillum, Venus flower basket, a 



"glass" sponge; Spongilla lacustris, a common fresh- water 



sponge; Hippospongia gossypina, an American species of 



commercial sponge. 



PHYLUM 3— COELENTERATA 



There are two morphological types among the Coelenterata: 

 (1) the polyp or hydroid form, which is sessile, and (2) the jellyfish 

 or medusoid form, which is free-swimming. The polyp type is 

 illustrated by Hydra, whose body is a double-walled, tubular sac 



