382 The Animal Kingdom 



One family of this class, the Spongillidae, contains the fresh-water 

 representatives of this phylum. These forms grow in loose masses on 

 suitable sticks and stones in waters that have sufficient organic material 

 for food. They reproduce both sexually and asexually and have de- 

 veloped gemmules to a great degree. They depend upon these for 

 surviving the unfavorable conditions of freezing and drying that ac- 



Fig. 125. — Examples of sponges. a, b, and c, Members of the class Demospon- 

 giae; d, a member of the class Hexactinellida. a, Stylotella; b, Spongia, the bath 

 sponge; c, Chalina; d, Euplectella, the Venus's flower basket. 



company a fresh water existence. These gemmules consist of masses 

 of amoeboid cells surrounded by a wall of spicules. Many of these 

 spicules are of a peculiar type, having a straight shaft with a small 

 wheel at each end. 



THE PHYLUM MESOZOA 



In their organization, the members of the phylum Mesozoa are the 

 simplest of all the multicellular animals. They have but two layers of 

 cells : an outer layer of ciliated cells surrounding an inner one of repro- 

 ductive cells. At this level of organization, these layers cannot be com- 

 pared with those of the Eumetazoa. 



These unusual animals are all endoparasites which live in the 

 internal organs or spaces of many different invertebrates such as the 

 squids, octopuses, flatworms, annelids, clams, and brittle stars. Most 

 have quite complex life cycles involving both sexual and asexual phases. 



