102 Mr. E. Ray Lankester on distinct Larval and 



X. — On the Existence of distinct LaYval and Sexual Forms 

 in the Gemmiparous OligochcBtous Worms. By E. Ray 

 Lankester, B.A. Oxon. 



In the ' Quarterly Journal of Microscopical Science ' for July 

 I have described the sexual form of Chcetogaster Limnceiy 

 which differs from the gemmiparous larvae abounding through- 

 out the year on the Limnoius and Planorhis in the fact that 

 the number of setae in each fasciculus is doubled, that there 

 are sixteen pairs of abdominal fasciculi instead of three or 

 four to each individual, that gemmiparity is discontinued, and 

 that a new pair of fasciculi developes between the cephalic aud 

 first abdominal pair of fasciculi, four set^ of which on either 

 side are not uncinated and bifid at the apex, but stunted club- 

 shaped organs. These I call " genital setse." In studying 

 the generative organs of Nats serpentina^ which swarms in a 

 very filthy pond on Hampstead Heath, which has furnished me 

 with Lumhriculus^ Limnodrilus (new species), Enchytrceus 

 (new species), and two other species of Nais^ I have ob- 

 served a somewhat similar change and development of " genital 

 setae," which do not appear to have been known to Carter 

 (who described the " spermatology " of a species oi Nais in 

 this Journal in 1858), nor to the late Jules d'Udekem (in his 

 description of Stylaria) ; nor have they been mentioned by 

 M. Edouard Clapar^de, to whom, however, I dare say they are 

 known, since he has studied a species oiNais, but, I believe, has 

 not published the description of it among his other invaluable 

 contributions to this branch of zoology. I therefore conclude 

 that these modified setse and their position are unrecorded 

 hitherto. 



In Nais serpentina and other species of Nazs, five pairs of 

 ventrally placed fasciculi succeed to tlie mouth, indicating a 

 pharyngeal region. There are no dorsal setse in this region. 

 Immediately after the fifth pair the intestine commences ; that 

 is to say, the alimentary canal is contracted and its walls are 

 covered by a layer of coarse cells — the so-called hepatic tunic. 

 Corresponding to the sixth pair of ventral fasciculi is a pair of 

 dorsal setae, thick, short, and awl-shaped in N. serpentina ; 

 these continue thenceforward, along the body, with the ven- 

 tral setae. The bristles of the ventral setae are curved, with a 

 hooked bifid apex ; two, with a growing third, is the usual 

 number in a fasciculus. When the generative organs com- 

 mence their development, the distance between the fourth and 

 fifth ventral fasciculi enlarges very considerably, and a new 

 pair of fasciculi makes its appearance, placed a little nearer the 

 middle line, and thei'efore closer together, than the other fasci- 



