13 J ELEMENTARY ZOOLOGY 



the insects and crabs. Almost all of the worms have the 

 power of locomotion ; usually that of crawling. For 

 this crawling they do not have legs composed of separate 

 segments or joints as do the higher articulated animals, 



FIG. 29. A group of marine worms; at the left a gephyrean, Dendrostomum 

 cronjhelmi, the upper right-hand one a nereid, Nereis sp., the lower 

 right-hand one, Polynoe brevisetosa. (From living specimens in a 

 tide-pool on the Bay of Monterey, California.) 



the crabs and insects, but either have fleshy unjointed 

 legs, or various kinds of bristles or spines, or suckers, or 

 even no external organs of locomotion at all. As regards 

 their internal structure they have well-organized systems 

 of organs, which show great variety in character and 

 degree of complexity. The special sense-organs are 

 usually of simple character and low degree of functional 

 development. Reproduction occurs both sexually and 

 asexually; in some species the sexes are distinct, while 

 in others both sperm-cells and egg-cells are produced by 

 the same individual. Asexual reproduction is by budding 

 or by a kind of simple division or fission. The worms 

 live either in salt or fresh water, or in moist, muddy or 

 slimy places or as parasites in the bodies of other animals 

 or in plants. While most worms feed on animal sub- 



