Sexual Forms in the Oemmiparous OUgochcetous Worms. 103 



culi. The bristles in these new fasciculi are shorter and 

 stouter than those of the normal ventral fasciculi, and are not 

 hifid at the apex ; they are three in number in each fasci- 

 culus, rarely fom*. At the same time that these make their 

 appearance, the normal ventral fasciculi acquire another bristle 

 in each, making thus three, with a growing fourth, or even 

 four complete, instead of two and three. The genital setce 

 which thus develope in the new genital segment are placed 

 close to the orifices of the male generative glands, the duct 

 connected with them being simple and unciliated. In front 

 of the fourth pair of ventral fasciculi are two orifices corre- 

 sponding well in position with the genital setge and their ori- 

 fices ; and these open into very large sacs, containing flat 

 rhombic crystals and curiously coiled fibrous whisps and sper- 

 matozoa — the seminal receptacles. The clitellus, which, as 

 all know who have studied the Oligochseta, is the most ob- 

 structive institution on account of its opacity, extends from 

 the fourth to the seventh fasciculus (not counting the genital 

 fasciculus in the numbering). 



I do not wish now to give a detailed account of the genera- 

 tive organs of Nais, which I hope shortly to publish else- 

 Avhere ; but I desire to call attention to this development of a 

 new segment between the larval fourth and fifth fascicular 

 segments, and provided with fasciculi carrying a special form 

 of genital set£e. For the greater part of the year these worms, 

 like Chcetogaster, reproduce gemmiparously under a certain 

 well-known form ; suddenly gemmiparity ceases, and a new 

 development, of which there was no previous indication, takes 

 place : a new segment, a new integral factor of the worm, 

 makes its appearance with a new form of setse ; the setaj in 

 the normal fasciculi also increase in number. This is not the 

 mere growth of generative organs occurring in due order of 

 development, but is really more strictly comparable to cases of 

 metamorphosis, the gemmiparous form being a larva, as the 

 agamogenetic Cecidomyia is a larva, and the sexual form the 

 perfect or imaginal condition. Very few of the immense num- 

 bers of gemmiparously produced Na'ides or Chcetogastres ever 

 proceed to acquire the perfect form, — Chcetogaster Lirnncei 

 only in October apparently, Nais serpentina in June, but per- 

 haps also at other seasons. 



There is, as far as I know, nothing described comparable 

 to this development of a new segment and of genital setge in 

 the other Oligoch^eta ; but, on inquiry, indications of it may 

 hereafter be found. The setae in the region of the clitellus in 

 Limnodriliis, in Tuhlfex^ in CliteUio, and others remain 

 normal, or drop out and are not replaced. 



