. Text book of zoology. Zoology. 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 o
. Text book of zoology. Zoology. 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-face 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 magnifi
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