174 



ZOOLOGY 



midst of a single worm. This habit is of advantage not only 

 in multiplying the numljer of individuals of the species, but 

 also as a means of protection. For if, b}' chance, the larva 

 of the water-beetle (Dytiscus) seizes a Nais and liites it in 



two, the part which es- 

 capes can go on devel- 

 oping new individuals. 

 The terrestrial Oligo- 

 chastae include the com- 

 mon earthworm and, in 

 addition, worms that 

 live on the seashore be- 

 tween the tides and 

 even in places whicli are 

 rich in vapors of am- 

 monia arising from de- 

 caying stuff. 



Tliere is another 

 group of fresh-water 

 worms which is closely 

 related to the Oligochseta but which has a A'ery different 

 appearance. The leeches, or " bloodsuckers," are flattened 

 worms, which, like the earthworm, show metamerism ; that 

 is, there is a repetition of the internal organs. They are also 

 segmented ; that is, the body has external rings, although 

 ihcy may be oltscure. One segment does not, however, as in 

 the earthworm, correspond with one set of internal organs 

 (metamere), but there are three, four, or five segments to a 

 metamere. Leeches have no appendages and no Ijristles ; 

 but they have a sucking disk at the posterior end of the body 

 for the purpose of sticking to things (Fig. 165). They usually 



Fig. 1C4. — Xais. a, mouth; h, anus; c, in- 

 testine. From Leunis. 



