188 PROCEEDINGS OF THE NATIONAL MUSEUM. tol. 48. 



frequently somewhat swollen and in mature specimens the sixth and 

 seventh form a clitellum. Paired or unpaired nephridial apertures 

 occur in segments 3 and 9 of the trunk. The spermatheca opens in 

 the mid- ventral line in segment 5; the male sexual aperture is sim- 

 ilarly located in segment 6. The female sex apertures, paired, open 

 on the sides of the posterior portion of segment 7. The anus is in 

 the mid-dorsal line of segment 10. 



The body cavity is divided by 7 dissepiments, forming the trunk 

 segments 1 to 8. 



The digestive cavity runs in a comparatively straight line from 

 the mouth to the anus which is on the dorsum just in front of the 

 sucker. The buccal region is provided with a powerful musculature 

 and armed internally with a pair of solid chitinous dental plates, one 

 dorsal and the other ventral, the dorsal being usually a little the 

 larger. When these plates are of different form there is a hiatus in 

 one to fit a tooth in the other. A muscular pharynx in the cephalic 

 segments is followed by a short esophagus in the first trunk segments, 

 passing without distinct demarcation into the intestine which termi- 

 nates in an anus with a muscular sphincter. 



The nervous system consists of a pair of supraesophageal ganglia 

 in the head region. Circumesophageal commissures from these unite 

 ventrally to form 3 pairs of simple or bilobed ganglia and 3 large 

 pairs of lateral nerve trunks, which indicate the tripartite segmenta- 

 tion of the head. Following this there is a chain of 8 ganglia, usually 

 double or bilobed, which becomes in the last 3 trunk segments a 

 ganglionic mass, corresponding, as its nerves show, to 3 pairs of 

 bilobed ganglia. 



The circulatory system consists of a dorsal and a ventral vessel, 

 connected anteriorly by 4 pairs of transverse trunks and posteriorly 

 by 1 pair. From the third to the eighth trunk segment the dorsal 

 vessel forms the pulsatile heart. The ventral vessel divides poste- 

 riorly into two branches which pass up around the intestine and 

 unite to join the posterior limit of the dorsal vessel. 



The excretory system consists of 2 pairs of nephridia. The ante- 

 rior pair has its ciliated apertures in the second trunk segment and 

 opens externally by lateral pores or a single dorsal pore just behind 

 the dissepiment separating segments 2 and 3. These nephridia are 

 not bilaterally symmetrical. The anterior tubule may extend from 

 the third into the first segment and the posterior from the third into 

 the fourth. The second pair of nephridia are symmetrical and are 

 in the eighth trunk segment with the nephridiopore opening exter- 

 nally just behind the dissepiment separating segments 8 and 9. 



The discodrilids are always hermaphroditic, the genital organs 

 being in the fifth, sixth, and seventh segments. The male organs 

 consist of a pair of testes in the anterior part of the fifth segment, 

 with sometimes another pair in the anterior part of the sixth, the 



