256 



ZOOLOGY 



SECT. 



The Trematodes nearly all have two testes, sometimes compact 

 sometimes branched ; in a few instances there are four. The vasa 

 deferentia unite into a median duct, which is dilated at the base 



of the penis to form a vesicula 

 seminalis. There is a single 

 oval or branched ovary, and 

 two sets of vitelline glands. A 

 canal termed Laurers canal 

 leads from the exterior to the 

 oviduct or V 7 itelline duct. 



In the ordinary Cestodes 

 each segment or proglottis con- 

 tains a set of reproductive 

 organs similar to those of a 

 Trematode. There may be a 

 single genital aperture leading 

 into a genital cloaca into which 

 both male and female ducts 

 open ; or the male and female 

 apertures may be distinct 

 though close together. The 

 testis is divided into a number 

 of minute lobes, from which 

 proceed a number of fine canals, 

 joining together to form the 

 vas deferens, at the extremity 

 of which is the chitinous cirrus. 

 There are two ovaries, and 

 either a single vitelline gland, 

 or two. One or two vitelline 

 ducts and the two oviducts open into the ootype, from which 

 the vagina leads to the external aperture. There is in the 

 Cestodes a uterus distinct from the ootype in the form of a diver- 

 ticulum of the latter into which the eggs are received ; this 

 becomes greatly enlarged, lobed, and branched in the " ripe " 

 posterior proglottides, so that it soon becomes by far the most 

 conspicuous part of the reproductive apparatus the rest eventually 

 becoming absorbed. In JBothriocepkalus and its allies the uterus 

 has an independent duct of its own, opening at some distance 

 from the common genital aperture ; in Caryophyllceus this uterine 

 duct joins the vagina. 



The development of some of the Platyhelminthes (Rhabdocoela, 

 Monogenetic Trematodes) is direct i.e., not complicated by the 

 occurrence of a metamorphosis ; in the Digenetic Trematodes, the 

 Cestodes, and some of the Planarians a metamorphosis occurs. 



The ovum of a Polyclad (Fig. 205) on impregnation divides 

 first into two equal parts, then into four. From each of these 



nt 



FIG. 204. Reproductive organs of Meso- 

 stomum Ehrenbergii. <'</. dm-r <if 

 vitelline glands ; rlo. vitelline glands ; n<>. 

 common reproductive aperture ; or. ovary ; 

 p. penis ; r.s. receptaculum seniinis ; .<. 

 pharynx ; t., t. testes ; ut. uterus ; ad. vas 

 detVivns. (From Clans, after von Graff 

 and Schneider.) 



