PLATYHELMINTHES 



217 



opening in front of it. The nervous system is of the primitive network 

 type, but the ovary and vitellarium are separate. Many authors place 

 the Temnocephalea with the Turbellaria, basing their claims to be 

 associated with this class rather than the Trematoda on the presence 

 of some scanty cilia, rhabdites, a basal membrane and the absence 

 of any chitinous thickening to the sucker and the absence of Laurer's 

 canal. They are symbionts rather than parasites, which further dis- 

 tinguishes from the Trematoda, but their thick cuticle and their 

 syncytial ectoderm are undoubtedly Trematodan in character. 





l-v£ sue 



CU. 



circ.in. 



I lovg.m. 



( ect.c. 



-par.c. 



— vesx. 



Fig. 156. 



Fig. 157- 



Fig. 156. Temnocephala minor, x 12. After Haswell. g.o. genital opening; 

 M. mouth; sue. sucker; ten. tentacles. 



Fig. 157. Transverse section through body wall of a trematode. After 

 Benham. ba.memh. basement membrane; cire.m. circular muscle layer; 

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

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



The rest of the Trematoda are all parasitic but they resemble in 

 general shape the Turbellaria. They have retained the mouth, which 

 is anteriorly placed, and the gut, which, however, is bifid, a shape not 

 found in the Turbellaria. As in the Turbellaria, the gut may have 

 lateral diverticula which branch freely. The Trematoda have, how- 

 ever, lost the external ciliation of the Turbellaria (Fig. 157). The 



