148 ZOOLOGY 



appendages on many of the segments. The segments are more 

 nearly homonomous than in typical arthropods. The earth- 

 worm, many types of aquatic worms, and leeches are included 

 here. 



185. Unsegmented Worms (embracing numerous ill-as- 

 sorted animals of doubtful relationship). Here may be in- 

 cluded a number of small groups many of which were once 

 classed with the Annelida and called "worms." They are not 

 sufficiently alike to be regarded as one distinct phylum. In- 

 deed there are probably three or four small phyla included. 

 The majority of them are bilaterally symmetrical, unsegmented 

 and without appendages. They differ from the rnoUusks in 

 that they do not possess a mantle and usually do not secrete 

 a shell. Many of them are parasitic. Among these animals 

 of doubtful relationship may be included the "flat- worms," 

 " thread- worms, " the Nemertinea, rotifers, and others. 



186. CoBlomata (with coslom) and Ccelenterata {hollow 

 inside). — ^All the animals thus far considered possess during 

 some stage of life a more or less developed body cavity or 

 coelom (see §58) distinct from the digestive tract. For this 

 reason they are sometimes known collectively as Ccelomata. 

 All the remaining many-celled animals have a general cavity 

 which serves both as a body cavity and a digestive tract {gastro- 

 vascular cavity), — or to speak more exactly, there is no true 

 body cavity. Of these the phylum Caslenterata is the chief illus- 

 tration. Here belong the jelly-fish, sea-anemone, and corals. 

 They are all aquatic and are more or less tubular, sac-shaped 

 animals often attached by one end, with the mouth, which also 

 functions as the anus, at the other surrounded by clusters of 

 tentacles. Many secrete skeletons, and some form immense 

 attached colonies. 



187. Phyliun Porifera {pore-hearing). — This group, to which 

 belong the sponges, is sometimes classed with the Coelenterata. 

 While similar to them in habit the sponges are much less highly 

 organized and unified. Instead of a single mouth opening into 

 the digestive tract, sponges have many openings or pores 

 (whence the name Porifera) which are the beginnings of tubes 



