DESCRIPTIONS OF GENERA AND SPECIES 281 



The body consists of four segments. The ventral nerve-cord has a large number of ganglionic 

 swellings, not always sharply marked oif from each other. 



Imhop regards it as nearly allied to Ctenodrilus, but remarks that the nervous system lies freely 

 in the coelom, and is not as in that genus imbedded in the body-wall. It differs, however, from 

 Ctenodrilus in having only two series of setae, and in this same character agrees with Chaetogaster. 

 I may point out also that the arrangement of the nervous system (more ganglia than apparent 

 segments) seems to agree with that of Chaetogaster. 



Genus Nais, 0. F. Muller. 

 Syn. Opsonais, Geevais. 

 Slavina, Vejdovsky. 

 Ophidonais, Gekvats. 

 Serpentina, Oersted. 

 Stylaria, Lamabok, &c. 

 Stylinais, Gervais. 

 ITereis, Linnaeus (in part). 



DEFlWiTiOlsr. Dorsal setae commence in segment VI, capilliform only, or capilliform 

 and uncinate, or straight with bifurcate end. Eyes present. 



I believe that we cannot separate the five 'genera' included in the above list of 

 synonyms. Bourne allows all of them, with the exception of course of Opsonais; 

 Vatllant, on the other, hand, merges Slavina in Nais, and Bousfibld includes 

 Ophidonais in Slavina. All the various species, which I here include under one 

 generic title, agree in the important fact that the first five segments are cephalized — 

 that the dorsal setae do not commence until the sixth segment. It is true that two 

 other undoubtedly distinct genera, viz. Bero and Ripistes, show the same character ; 

 but the former of these is distinguished by the remarkable branchial apparatus at the 

 tail end of the body, while Mipistes shows a more advanced degree of cephalization 

 in that the ventral setae also are missing from segments iv, v. The genus Slavina 

 of Vejdovsky only differs from Nais in two points : in the fact that the dorsal setae 

 of the first bundles are much longer than those which follow, and in the presence 

 of the cutaneous sensory papillae. As regards the first point, it may be mentioned 

 that similar difierences occur in the species of the genus Pristina ; the sensory 

 papillae seem to be wanting in S. gracilis of Leidy, which Vejdovsky associates with 

 his genus Slavina; in refusing to regard these structures as of generic value I am 

 in accord with Stolc and Bourne, who blame Bousfield for allowing this character 

 to have led him to associate together Slavina and Ophidonais. At anj' rate, if these 







