CHAPTER VII 

 SURVEY OF INVERTEBRATES 



It seems as if Nature had essayed one after the other every possi- 

 ble manner of living and moving, as if she had taken advantage of 

 every permission granted by matter and its laws. — Gide. 



The stupendous group of multicellular animals constituting 

 the Invertebrates presents a bewildering array of forms adapted 

 to nearly every conceivable environment. Some are smaller than 

 many of the Protozoa and others much larger than certain Ver- 

 tebrates. Some are aquatic, others are terrestrial, and still others 

 spend most of their life in the air. Most possess the power of 

 locomotion, but many are fixed, or sessile. And not a few — per- 

 haps the majority — live as parasites in or on the bodies of other 

 animals. (See Appendix I.) 



The fact that there are nearly a million known species, and 

 probably twice as many yet to be discovered, at once suggests that 

 our survey must be confined to a relatively few representatives 

 from each of the major Invertebrate phyla; enough to place the 

 three forms we have selected for special study later — Hydra, 

 Earthworm, and Crayfish — in proper perspective. 



A. Sponges 



One is not surprised that the Sponges which constitute the 

 Porifera, the lowest phylum of multicellular animals, show certain 

 relationships with the Protozoa. Specialization of cells and physio- 

 logical division of labor, though carried far beyond the most com- 

 plex colonial forms, such as Volvox, nevertheless have not sup- 

 pressed the individualities of the cooperating cells to the extent that 

 occurs in most higher forms. Witness the fact that when certain 

 Sponges are gently squeezed through the meshes of fine silk cloth, 

 so that the tissues are resolved into separate cells, these cells will 

 gather together in small groups and each group will grow into a 

 Sponge. In brief, the Sponge is an individual animal, but its 

 organization is loose and the dependence of one part upon another 



67 



