THE ANIMAL KINGDOM 



387 



and various other shapes. The cells of the mesoderm or more 

 properly, the mesenchyme, consist of (1) scleroblasts, which secrete 

 the spicules, and spongioblasts, which produce the spongin; 

 (2) stellate connective tissue cells; (3) myocytes, which are con- 

 tractile and are found at the pores and osculum, where they 

 function as sphincters; and (4) archeocytes. The last are 

 amoeboid cells that share with the choanocytes the ingestion 

 and digestion of food and also give rise to germ cells and to 



Fig. 218. — Portion of cross section of Grantia, a Sycon type of sponge, cc, 

 choanocytes; ect, dermal epithelium; fl, flagellum of choanocyte; mes, mesoglea; 

 sp, portion of spicule. (From Shull, LaRue, and Ruthven, Animal Biology.) 



gemmules. Reproduction occurs asexually by budding, sexually 

 from fertilized eggs and by the formation of gemmules. Gem- 

 mule formation, which is more common in fresh-water sponges, 

 seems to be an adaptation for surviving low temperature or lack 

 of water. This is accomplished, in the case of the fresh-water 

 sponge during summer and fall by the formation of capsules of 

 archeocytes in the mesenchyme. The gemmules, composed of 

 encapsulated archeocytes, fall to the bottom if the sponge dies 

 and remain there until the following spring, when they develop 

 into sponges. All fresh-water sponges do not die in the winter, 

 even though gemmules are formed. 



