278 OLIGOCHAETA 



The vascular system in the genus Nais itself has been principally studied by 

 STOLC and VEJDOVSKY. It is only in N. josinae that the dorsal vessel has a series 

 of lateral arches in all the segments of the body ; in the other species there 

 are only a few such vessels in the anterior segments. In N. elinguis, for example, 

 there are three pairs of lateral vessels ; these are quite simple ; a fourth is sometimes 

 present ; three pairs of lateral vessels also occur in N. barbata, N. lacustris, and 

 Naidiuni luteum ; in N. lacustris these vessels are in segments iv, v, vi ; in Naidium 

 in the same segments ; Priatina longiseta has four pairs of equally simple perienteric 

 vessels in segments v, vi, vii, viii ; Priatina offers a further degree of complication in 

 that the last pair are dilated. 



N. serpentina exhibits another kind of complication, a first step, which is after- 

 wards more fully developed ; there are three pairs of trunks as in most other Naids, 

 but the first of these are dichotomously divided ; in Bohemilla the same thing occurs, 

 but the first pair of vessels is not only once dichotomously divided, but again. 

 In N. heterochaeta all of the lateral vessels give off branches which join the successive 

 arches, and often make more than one connexion with the ventral vessel ; this will 

 be found more fully described below, under the description of that species ; finally, in 

 N. josinae the anterior lateral vessels are hardly recognizable, as they form an elaborate 

 network. In Uncinais littoralis there are still more pairs of lateral vessels ; five 

 pairs in all are figured by BOURNE (5) in addition to the terminal bifurcation of the 

 dorsal vessel to join the ventral vessel ; of these the first two and the terminal 

 bifurcation anastomose by several branches ; these lateral vessels end in the seventh 

 segment. In Chaetogaster the dorsal and ventral vessels are only united by one pair 

 of lateral vessels ; the communication elsewhere is effected by paired branches from 

 the network upon the intestine. 



It has been already mentioned that N. josinae has a pair of commissural vessels in 

 every segment of the body ; the same state of affairs is met with in the genus Dero. 

 The circulatory system of this Annelid had been studied by PERKIER, STOLC, and 

 by myself. As compared with the other Naidoniorpha, Dero is very highly organized 

 in its vascular system ; a variable number of the anterior lateral vessels are contractile ; 

 I found that six were so in Dero perrieri ; four (BOUSFIELD) is a more usual number. 

 STOLC has described a network formed by the first few lateral vessels in Dero (? sp.), 

 similar to that of N. josinae. 



In both Nais and Dero the intestine is invested by a close network of capillaries ; 

 this network takes its origin from the dorsal vessel, and communicates with the 

 ventral vessel by a series of median unpaired vessels, one to each segment. 



The reproductive organs in the Naidoniorpha have been examined in a few species 



