THE TREMATODA 



regarded as a retina, and Goto has suggested that they serve in 

 the Heterocotylea as " organs of temperature." 



The generative system presents a somewhat greater com- 

 plication than in the most elaborate Turbellaria, but is built up 

 essentially on the same plan ; the female gonad being, except in 

 GfyrodactyluS) composed of germarium and vitellarium. The male 

 and female ducts, with rare exceptions, open together into an 

 " atrium genitale," and the penis is frequently armed with spines. 

 In the Malacocotylea the original pair of testes persists, and with 

 very few exceptions, such as D. hepaticum, each testis is a com- 

 pact, rounded body. In the Aspidocotylea and Heterocotylea one 

 of the original pair has disappeared, the remaining gland which is 

 provided with only one duct, may remain compact (Fig. IY. 2), or 



FIG. XXI. 



Nervous system of Tristomum 

 molae (after Lang), as type of that 

 of Heterocotylea, viewed from the 

 ventral surface, a, brain (on which 

 rest the four eyes indicated by 

 white dots) ; b, dorsal nerve stem ; 

 c, lateral ; d, ventral. The ventral 

 steins are united by a series of com- 

 missures, which are continued on 

 to the lateral nerve, 13-15 in num- 

 ber (g). From the brain, on each 

 side, a nerve goes to " prostom- 

 ium," a second to the sucker. 

 These are joined by a ring-like com- 

 missure, arising from the stem 

 common to the posterior nerves. 

 The lateral and ventral nerves unite 

 in the posterior sucker and are 

 connected by a couple of semicir- 

 cular commissures, and give rise to 

 network in wall of sucker. Ji, 

 marginal network of body ; e, lateral 

 anterior sucker ; /, posterior sucker ; 

 /', radii. 



become subdivided by ingrowths of connective tissue into a few 

 large lobes (Epibdella), or more generally into many small ones 

 (Fig. II. 2; V. 2), so as to assume the "follicular" condition 

 present in many Turbellaria. 



The male copulatory apparatus varies considerably in details 

 of its structure, but these may all be reduced to two types : (1) 

 The word " cirrus " is used for the terminal, eversible part of the 

 sperm duct of Distomum and others which projects from the 

 bottom of the male antrum, and is enveloped in the " cirrus sac," 

 containing glands ; booklets are borne along the wall of the duct, 

 and on eversion come to project outwards. A cirrus is rare 

 amongst the Heterocotylea, being met with in Tristomum and 

 Epibdella. (2) A "penis," such as occurs in the Holostomidae, 

 and in the majority of the Heterocotylea, and in Aspidocotylea, is 

 a specialisation of the terminal region of the sperm duct which 



