Platyhelminths. Glass 1. Turbellaria. 



143 



usually open in the hind region of the body by a single or double 

 aperture : occasionally several openings are present. The principal 

 tube sometimes exhibits a contractile enlargement just in front of the 

 aperture (urinary bladder). The finest terminal branches of the 

 canal-system are especially characteristic ; each ends in a little cup- 

 shaped swelling, closed by a large cell (flame cell), which bears, 

 on the side turned towards the lumen of the tube, a very powerful 

 flagellum. Similar flame cells may also be found at other points in 

 the wall of the tube. The male and female genitalia (Fig. 99) 

 are usually present in the same individual, and are, as a rule, of a 

 very coniplicated structure ; testes and ovaries are often present in 

 great numbers, and, besides these, various organs accessory to each 

 system, as also a uterus. The genital aperture is usually ventral. 

 Grenerally there is a copulatory organ. 



Amongst the accessory sexual organs, the yolk- gland (vitellarium), 

 which is of very common occmrence, may be specially noticed. In it the 

 so-caUed yolk-cells are formed, to be enclosed with the ovum in the egg 

 membranes, and to serve later as food for the embryo. Shell-glands are 

 generally present; their secretion, when hard, fonning the fii-m shell smrounding 

 the eggs of many forms. 



A. Without anus or vascular system. Hermaphrodite with complicated 

 genitalia. - 



1. Turbellaria. As a rule free-living, sm-f ace ciliated, alimentary 

 canal generally present. 



2. Trematoda. Parasites, without cUia. Alimentary canal 

 present. 



3. Cestoda. Parasites, without cilia. Alimentaay canal ' always 

 absent. As a rule form chains. 



B. With anus and vascular system. Separate sexes. Genitalia simple. 



4. Nemertina. 



Class 1. Turbellaria. 



The Turbellaria are animals of varying, but usually small, size. 

 The body is uniformly ciliate, which is important for locomotion 

 as well as for respiration ; some are active, many magnificently 



Fig. 97. Longitudinal section through a TurbeUarian (Cyclop'orus papillosiis), to show 

 the situation of the pharynx. Of the organs caught in section only the gut is figured. 

 m mouth, p retracted pharynx, r dorsal papilla, s sucker, t gut-; 3 male, $ female 

 aperture, enlarged. — Adapted from Lang. 



