GENERATIVE ORGANS OF VERMES. 183 



A large number of unicellular glands are attached to the point 

 where the ducts of the yelk-gland and the oviduct unite, in Trema- 

 toda (Distoma, Polystomum, Amphistoma) and Cestoda (Bothryo- 

 cephalus, Tamia). This group of glands is known as the shell-gland, 

 and its secretion serves to form the investment of the ova (Fig. 87, <jl). 

 In addition thei'e is in the Bothryocephali and many Trematoda a 

 special canal at this point, which opens either into the sinus genitalis, 

 as in the former, or on the dorsal surface of the body, as in Distoma 

 hepaticum : it evidently functions as a vagina, as it has been found 

 full of sperm (Fig. 87, v). This second communication between the 

 female organs and the exterior makes it possible for impregnation 

 to take place without interfering with the gradual evacuation and 

 deposition of the ova. In this double opening of the female organs 

 we may perceive an indication of the primitive double nature of 

 the whole system. 



The influence of a change in the external conditions of life upon 

 the genital apparatus in the case of Polystomum (P. integerrimum) 

 furnishes an instructive example of the capacity for adaptation of 

 an organ in full function, and therefore presumably mature. 

 The change is one of abode, and results in an increased pro- 

 duction of generative matters, and the correlative appearance of new 

 parts in the generative apparatus. 



§ 150. 



We know as yet very little as to the way in which the herma- 

 phrodite apparatus acts in copulation. In many cases there are 

 arrangements favourable to self-impregnation. 



The genital pores differ in position in the various divisions of 

 the Platyhelminthes. In most cases the generative organs open in 

 the ventral median line, sometimes very far forwards, just behind 

 the oral sucker, as in many Trematoda (Distoma, Gyrodactylus, etc.), 

 and sometimes nearer the hinder end of the body (Turbellaria), or 

 even quite at that end (Distoma macrostomum). Among* the Cestoda 

 also the ventral position is not uncommon (Ligula, Bothryo- 

 cephalus) ; in most cases the genital pore is a flattened depression 

 on the lateral edge of the proglottides, placed alternately on one 

 and the other lateral edge. The fact, that in some Cestoda (Tamia 

 elliptica, T. cucumerina) there are two symmetrically-disposed gene- 

 rative systems in each proglottid, is important as bearing on the 

 meaning of this want of symmetry, which obtains even in some Trema- 

 toda (Tristoma). The unilateral condition may be regarded as the 

 remains of a primitively double arrangement; it was, then, only 

 gradually that the system of one side got to predominate over the 

 other, and led to that relation of parts which is now the most widely 

 distributed — the unilateral development of the generative system. 



There is, with but few exceptions, only a single genital pore 

 in the rhabdoccelous Turbellaria, the male and female organs 

 both leading to it. In the dendroccelous forms the orifices become 



