270 ZOOLOGY sect. 



separated off and does not in most cases become renewed, though 

 in at least one species of Tape-worm {Tamia ciicumerina), a new 

 vesicle is developed again and again at the end of the body as a 

 fresh segment is thrown off. The main trunks are connected 

 together by a ring-vessel in the head and in some cases by a 

 transverse branch in each proglottis, and where the latter originate 

 from the main trunks are valves formed by folds of the wall of the 

 vessel. In the posterior region only two of the longitudinal 

 trunks (one on each side) may be retained. 



The sexes are united in all the Platyhelminthes with only 

 one or two exceptions, and the reproductive organs are 

 sometimes somewhat complicated — presenting a remarkable ad- 

 vance on those of the Ccslenterata. The male part of the 

 apparatus consists of testes, ^vith their ducts, the vasadeferentia, often 

 with a contractile terminal enlargement or vesicula seyninalis, a 

 cirriis'- or a penis, and often prostate or granule-glands. The female 

 part comprises ovary or ovaries, receptaculum seminis, oviduct, uterus, 

 an ootype, often a bursa copulatrix, sliell- glands, vitelline or yolk- 

 glands, and cement- glands. In most, though not in all, there is a 

 single or paired germarium' (ovary), in which the ova are formed, 

 and a set of viiellaria or vitelline glands, producing material which 

 surrounds each of the mature impregnated ova before it becomes 

 enclosed in its shell. In some, on the other hand, ova and vitelline 

 matter are formed in the same organ — the germ-vitellarium. The 

 shell-glands are so named because they are usually supposed to 

 secrete the chitinoid substance of the egg-shells ; but the 

 share which they take in this process is uncertain. The 

 cement-glands secrete a viscid material for causing the eggs to 

 adhere together, enclosing them in a cocoon or fastening them to 

 some foreign body. The oviduct is the passage by which the ova 

 reach the exterior from the ovary ; but an enlarged part of this 

 passage, into which ducts of the shell glands open, is distinguish- 

 able as the ootype, while a terminal part, leading to the female 

 aperture may be modified as a vagina. In some cas.es (Heterocoty- 

 lean Trematodes) there is a vagina or a pair of vaginae in the shape 

 of a passage, or a pair of passages, distinct from the oviduct and 

 opening independently on the exterior. A uterus in the form of an 

 enlarged part of the oviduct or of an outgrowth from the latter or 

 from the atrium is very usually developed for the reception of 

 the completed eggs. A special sac or bursa copidatrix, lined with 

 spines, acts as the female copulatory organ. A sac, the receptaculum, 

 opening into the oviduct or into the atrium (Figs. 201, 202, r.v.), 

 may serve as a reservoir for the semen received in copulation 

 or for the vitelline matter or yolk, or for surplus reproductive 



' The term cirrus is here restricted to cases in which the terminal part of the 

 male duct, often provided with spines and other chitinous structures, is 

 involuted within a sheath when at rest. 



