380 The Animal Kingdom 



and the flagellated cells come to lie on the outside. This is known as 

 the amphiblastida stage (Fig. 123,C), and it is this larval form that es- 

 capes from the parent through the osculum. This swims about for a 

 short period. The flagellated portion of this larva either invaginates or 

 perhaps is overgrown by the larger nonflagellated cells, and the larva 

 attaches to a solid object. The mesenchymal cells then arise from both 

 layers. This complex embryology is the reason why it is difficult to 

 homologize the germ layers of the sponge with the more clear-cut ones 

 of most other metazoans. 



Fig. 124. — Sponges. A, Types of spicules; B, section of gemmule 

 of fresh-water sponge. 



The Biology of the Sponges. — In spite of their seeming simplicity 

 of construction, the sponges are a very successful group both from the 

 viewpoint of the number of individuals and their worldwide distribu- 

 tion. They attain their greatest size, diversity, and number of species 

 in the warm ocean waters, but they are also found in the colder ocean 

 waters and often attain considerable abundance in bodies of fresh 

 water. Within their habitat, they play an interesting role. Due to 

 their being sessile and having many branched portions, they are the 

 hiding place for many difi'erent animals. For example, many small 

 shrimps and fish abound among the branches. Worms, echinoderms, 

 coelenterates also may be found on the surface and at times within the 

 canal system. One particularly unusual relationship is that of a small 

 shrimp found in the waters ofif Japan. When these are very small, a 

 pair enters the spongocoel and grows in size. It is of course unable 

 to escape and remains permanently imprisoned within the sponge. 



Some crabs actually tear ofif pieces of sponge and attach them to 

 their backs for camouflage. Mutual benefit is derived from this, for the 



