TREMATODA. 227 



yolk. This view is not, however, borne out by an examination of its contents, 

 which are often spermatozoa, though sometimes yolk, and even ova. Some 

 observers hold that the single canal of the digenetic forms is not homologous 

 with the two canals of many monogenetic forms, but with the vitelio -intestinal 

 canal, which is not present in the Digenea. 



On the whole we incline to the view that the canal of Laurer, whether 

 double or single, is in all cases homologous, and serves for the entrance of 

 spermatozoa to the female ducts, when, as is often the case, the uterus is full 

 of ova. It may be that in some cases the penis is used, like that of the 

 Turbellaria, for hypodermic injection of spermatozoa ; but as a general rule the 

 thickness of the cuticle and the isolation of the individuals will prevent this ; 

 again, sometimes the penis may, more or less accidentally, find its Avay into the 

 opening of its own uterus, or into that of another worm ; but, as a general rule, 

 the parasites being not contiguous, and having but little power of locomotion, 

 the spermatozoa are discharged into the cavity in which the parasite is lodged. 

 The testis being very large, the number of spermatozoa so discharged is probably 

 very great, so that they spread over the whole surface of the infected organ, to 

 which other parasites of the same species are affixed. Some of them, therefore, 

 are sure to reach the body of other individuals, and to pass, by their own move- 

 ments, into the opening of the canal in question. 



That some kind of haphazard impregnation of this kind is the rule in Trematoda 

 is rendered almost certain by the relatively enormous size of the testis. On 

 this view the penis must be regarded as a vestigial organ, with little or no 

 function under the present conditions of life. 



Although the so-called internal or third vas deferens does not exist, self- 

 impregnation* seems in some cases to occur. For in some species solitary 

 specimens have been found with spermatozoa in the female passages (e.g., 

 Polystomum integerrimum, Distomum agamos encysted in Gammarus pulex). 

 This self-fertilization may have been effected by the insertion of the penis into 

 the opening of the uterus, but probably it has more often been effected in the 

 manner above suggested. 



The egg-shell is usually in two parts, a small piece being 

 separated by a suture from a larger, and constituting the oper- 

 culum. In the ectoparasitic forms it is often prolonged into a 

 filament at one or both poles of the egg, which filament serves for 

 the attachment of the egg (Fig. 188). Filaments are only rarely 

 found in the endoparasitic forms. 



The eggs of the endoparasitic forms are smaller and more 

 numerous than those of the ectoparasitic division. The early 

 stages of development are sometimes passed through in the uterus, 

 but in some cases the segmentation does not begin till after ovi- 

 position. Most Trematodes lay their eggs, but a few are viviparous 

 (Gyrodadylus). 



The just hatched young either possess (in most ectoparasitic forms, 

 Heterocotylea) the form and organisation of the parent (Fig. 186), 



* A. Looss, "Beitrage z. Kenntniss der Trematoden," Z. f. w. Z., 41, 185, 

 p. 420-427. 



