io8 



SYSTEMATIC ZOOLOGY 



the whole structure of the body. Often the mouth does not 

 open directly into the digestive portion of the alimentary canal, 

 but instead into a pharynx, which may sometimes be protruded 



to a considerable extent through 

 the mouth. The excretory 

 system consists of numerous 

 branched tubules which open in 

 various ways to the outside. 

 The reproduction is usually 

 sexual, and in some cases 

 curiously shaped larvae are pro- 

 duced. In some few species 

 asexual reproduction occurs by 

 a process of transverse division; 

 the individuals formed in this 

 way usually remain connected 

 with one another for a short 

 time, so that they look like a 

 worm made up of a series of 

 similar segments, or like a 

 colony of worms (Fig. 99); but 

 eventually they all separate and 

 each becomes a distinct, com- 

 plete individual. 



CLASS II. TREMATODA 



The Trematoda (Gr. rprjfiarco- 



FlG. 98. General struciure of a Turbella- 

 rian. (The male reproductive organs have 

 been omitted from the right side of the dia- g 7?9) having holes) resemble the 



gram, the greater portion of the female from 



Turbellaria with such differences 

 as are brought about chiefly 

 by a parasitic life, for all the 

 representatives of this class 



the left.) en, brain; e, eye; g, ovary; /,, 

 median limb of intestine; /. 2 , left limb; 

 i s , right limb; hi, longitudinal nerve-cord; 

 m, mouth; od, oviduct; />//, pharynx; /. 

 testes; te, tentacles; v, vitelline or yolk 

 glands; vd, vas deferens; u, uterus; 



penis; v, vagina; fQ, common genital are parasites, and nearly all are 

 opening. (After von Graff, from Parker hermaphroditic. There are both 



and Haswell s Manual.) ' 



ectoparasites, living on the out- 

 side of other animals, and entoparasites, living within other ani- 

 mals. Cilia are absent from the surface of the body in the 

 adult, and eyes occur in some of the ectoparasites only, while 



