66 MORPHOLOGY OF INVERTEBRATE TYPES 



nerves. Two of these six nerves are ventral, two dorsal and 

 two lateral. Their ramifications supply with nerves the vari- 

 ous organs. The six nerves running backward anastomose freely 

 with each other by means of numerous transverse commissures. 

 Ganglionic cells are found throughout the length of the main 

 stems. The mouth sucker and the pharynx receive each a 

 pair of special nerves from the cerebral ganglia. Special sense 

 organs are absent. 



No circulatory or respiratory system is present. 



Reproductive system. D. lanceatum is a true hermaphro- 

 dite like the great majority of the Trematodes, i. e., it has a 

 complete set of male and female sexual organs in the same in- 

 dividual. The genital openings are separate, both on the ventral 

 surface in the median ventral line. The female genital opening 

 lies in front of the acetabulum and about on the same level with 

 the two branches of the intestine where they merge in the 

 oesophagus. The male genital opening lies immediately behind 

 the female opening. A common genital cloaca, so common in 

 Trematodes, is wanting in D. lanceatum. The male genital 

 apparatus consists of a pair of testes, their ducts and an organ 

 of copulation. The testes are situated between the two branches 

 of the intestine. They are not symmetrical in their position. 

 The first testis lies immediately behind the acetabulum, while 

 the second testis lies somewhat to the left immediately behind 

 the first one. Accordingly the ducts or vasa deferentia are not 

 of equal length and since both run forward, that of the posterior 

 or second testis is the longer. Both ducts meet in the median 

 line immediately in front of the acetabulum. From here on 

 they continue as a single duct or ductus ejaculatorius. This 

 duct is inclosed in a so-called cirrus pouch and presents three 

 sections, the seminal vesicle, the median or prostatic section and 

 the terminal or cirrus section. The latter may be protruded 

 through the male genital opening by a process of evagination or 

 turning inside-out when it functions as the cirrus or male organ 

 of copulation. The walls of the cirrus have circular and longi- 



