192 



THE INVERTEBRATA 



another at certain stages in their life history. In the Heterocotylea the 

 excretory pores are paired and lie near the anterior end of the body, 

 whereas in the Malacocotylea the excretory system discharges to the 

 exterior through a single median pore placed at the posterior end of 

 the body. In the Heterocotylea there are separate openings for the 

 male and female portions of the generative system, while in the 

 Malacocotylea there is but one common opening. In the Heterocotylea 

 there is a pair of ducts leading from the ootype to the exterior indepen- 

 dently from the male and female ducts, usually called the vaginae. 



en. 



'= — ~ha.mcmb. 



pane. 



Fig. 151. Transverse section through body wall of a trematode. After 

 Benham. ha.ynemb. basement membrane; circ.jyi. circular muscle layer; 

 cu. cuticle; ect.c. ectoderm cell; long.m. longitudinal muscle layer; par.c. 

 parenchyma cell; 5^. spine; ves.c. vesicular cell (present in many trematodes). 



They are inconspicuous as a rule, but in Polystomum their openings are 

 very clearly marked by two prominences on either side of the body 

 about one-fifth of the body-length from the anterior end (Fig. 152). 

 Corresponding ducts do not occur in the Malacocotylea. The nervous 

 system of the Heterocotylea is more primitive than that of the Malaco- 

 cotylea, but in both groups it is stereotyped and does not vary as it 

 does in the Turbellaria. In both groups it consists of a cerebral 

 ganglion with six cords leading posteriorly. In the Heterocotylea 



