GENERAL PREVIEW OF THE ANIMAL KINGDOM. 139 



Usually a skeleton is developed in the skin. This is often cov- 

 ered with spines, and from this fact the phylum has its name. 

 They are marine and are poor movers, a few being fixed by 

 stalks to objects in the ocean. The starfish, sea-urchin and 

 sea-lilies are representatives. 



179. Annulata (Segmented Worms). This phylum is 

 similar to the arthropods in that the body is bilaterally sym- 

 metrical, is segmented, and has paired appendages to many of 

 the segments. It differs from them in the fact that the append- 

 ages, when present, are not jointed but are merely setae or hairs 

 in sockets or on fleshy prominences. The segments are more 

 nearly homonomous than in typical Arthropods. The earth- 

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

 here. 



1 80. Unsegmented Worms (embracing numerous ill- 

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

 cluded a number of small groups many of which have long 

 been grouped with the Annulata and called " worms." They 

 are not sufficiently alike to be regarded as one distinct phylum. 

 The majority of them are bilaterally symmetrical, unsegmented 

 and without appendages. They differ from the Mollusks in 

 that they do not possess a mantle and do not secrete a shell. 

 Many of them are parasitic. Among these animals of doubtful 

 relationship may be included the " flat-worms," " round- 

 worms," the nemertea, rotifers, and others. 



181. Coelomata and Coslenterata. All the animals thus 

 far considered possess during some stage of life a more or less 

 developed body cavity or ccelom (see 56) distinct from the 

 digestive tract. For this reason they are sometimes known 

 collectively as Ccelomata. All the remaining many-celled ani- 

 mals 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 Ccelenterata are the chief illustration. Here belong 

 the jelly-fish, sea-anemone, corals. They are all aquatic and 



