Chap. XIII. ] GONADS CF CtSTODA. 485 



persons, more or less medusoid in form, devote them- 

 selves solely to the duty of producing genital glands, 

 and obtain the necessary food from the nutrient 

 persons of the colony ; in a few cases these gonosomes 

 become free. 



The Platyhelmintnes present an elaborate 

 and somewhat difficult arrangement of their sexual 

 organs ; this is no doubt to be partly explained as due 

 to their exhibiting an early stage in the consolidation 

 of the diffused reproductive cells ; we must, how- 

 ever, not fail to note that they present a distinct 

 advance in the possession of accessory repro- 

 ductive organs. The male is provided with a 

 copulatory organ or penis, and the female, which 

 may now have special ovarian ducts, has the ter- 

 minal portion of the efferent tube modified into a 

 special canal (vagina), into which the male organ 

 may be received. Nor is this all ; another portion 

 of the duct is widened out into a receptacle in which 

 the ova may pass through the earlier stages of their 

 development (uterus) ; and yet another is often 

 converted into a pouch, in which the male elements 

 may be stored till such time as the ova are ready for 

 fertilisation (receptaculum seniinis). The egg- 

 producing and the yolk-producing cells are, however, 

 still distinct, and the latter have not yet, as in the 

 case of a bird, for example, taken their place on some 

 part of the duct that leads from the ovaries. 



This kind of arrangement is well seen in the Cestoid 

 Bothriocephalus latus (Fig. 204; AB), where the testes 

 (t) are seen to be represented by aggregations of cells 

 which are scattered through each segment j their pro- 

 ducts pass by narrow ducts (ve) into a common coiled 

 efferent vessel (vd) which opens at the anterior end of 

 the segment into the copulatory organ (cirrus, c). The 

 ovarian region (ov) occupies either side of the middle 

 line in the hinder region of the segment, while the 



