22 ELEMENTARY ZOOLOGY. 



30. Unsegmented Worms. Under this name we may include a 

 number of small often microscopic animals which do not belong 

 to any of the well-recognized phyla of animals. They are bilaterally 

 symmetrical, for the most part; they are not segmented, and do not 

 have the paired appendages. Many of them are parasitic ; but others 

 live in stagnant waters, and some of these are likely to be found at 

 almost any time in preparations standing in the laboratory. Here 

 may be included the "tape- worms" and other " flat- worms "; 

 trichina and other " thread- worms "; "wheel- worms," etc. 



31. Coelenterates. This group is wholly aquatic and chiefly 

 marine. It includes the jelly-fishes, sea-anemones, corals, and other 

 polyps. They are more or less tubular, sac-shaped animals, often 

 attached by one end. The mouth, which is at the free end, is 

 surrounded by a cluster of tentacles. The digestive tract has no 

 other opening than the mouth. Many of these animals secrete limy 

 skeletons, and some form immense attached colonies by their power 

 of budding the young individuals retaining their connection with 

 the parent stock, somewhat as the new branches of a tree are related 

 to the old ones. The internal structure of the ccelenterates is not so 

 complex as that of the groups mentioned above. 



32. Porifera. This group, which includes the sponges, is some- 

 times classed with the ccelenterates. While similar to them in 

 general habit, the sponges are much less highly organized and 

 unified. Instead of a single mouth, opening into the digestive 

 tract, the sponges have many openings or pores (whence the name 

 Porifera) which are the beginning of tubes carrying the currents of 

 water to a central cavity. This cavity communicates with the 

 exterior by means of one or more large outlets. The sponges are 

 mostly marine, are attached, and form large colonies by budding 

 and remaining together, as do the corals. The ordinary sponge 

 of commerce is the skeleton of such a sponge, after the fleshy or 

 living part has been removed. 



33. Protozoa. All the preceding phyla of animals consist, in the 

 adult stage, of individuals made up of many cells among which there 

 is more or less differentiation into tissues of several kinds. In all of 

 them the individual passes, in becoming adult, through stages in 

 which the cells are arranged in two layers (ectoderm and entoderm ; 

 see p. 121), and from these layers all the tissue masses arise. All 

 such animals are known as Metazoa. In the remaining phylum 

 the Protozoa the adult is a single cell, or at most a loose and poorly 



