68 MORPHOLOGY OF INVERTEBRATE TYPES 



duct. The Laurcr's canal is several times longer than the ovi- 

 duct. It runs forward at an angle to the longitudinal axis of 

 the body and opens on the dorsal surface considerably to the 

 left of the plane of symmetry, on the level of the posterior edge 

 of the second testis. Its function is as yet not well understood. 

 The uterus is in direct continuation with the oviduct and from 

 this it is evident that the duct of the receptacle and the Laurer's 

 canal mark the end of the oviduct and the beginning of the 

 uterus. In many Trematodes the space at the juncture of the 

 four canals is considerably widened and is then known under the 

 name of "ootype." But in D. Lanceatum no such ootype is 

 present. The uterus is a very long duct, about twelve times 

 longer than the animal itself and is consequently coiled up. The 

 diameter of the uterus at its beginning is no wider than the 

 diameter of the other three ducts but it becomes very soon con- 

 siderably wider. The course of the uterus is at first backward, 

 till it reaches close to the posterior end of the body. From here 

 on it runs forward, passes the testes and ends in the female 

 genital opening which has been already mentioned. The coils 

 of the descending (backward) portion of the uterus run parallel 

 with and ventral to those of the ascending portion. This is 

 the reason why in an animal stained in toto the uterus appears 

 as if it were branched. When fully developed the uterus oc- 

 cupies the entire space between the two branches of the intestine 

 and almost all the space to the rear of them. The two yolk 

 glands are situated in the middle third of the body to the out- 

 side of the intestinal branches. Each gland is in reality a com- 

 posite of a number of small glands which open separately into 

 the longitudinal yolk duct (of which therefore there are two, one 

 on each side of the body). The longitudinal yolk ducts are 

 connected with the transverse yolk ducts which run approximately 

 at right angles with the axis of the animal, meet in the middle 

 and form a short common yolk duct which opens into the uterus 

 close to the duct of the receptacle. The transverse yolk ducts 

 are ventral to the intestinal branches. Mehlis' gland consists 



