304 OLIGOCHAETA 



important difference between Amphichaeta and Chaetogaster is the presence in the 

 former genus of sperm-sacs and egg-sacs ; these are situated dorsally to the intestine, 

 and consist of outgrowths of the anterior septa of the testicular and of the ovarian 

 segments ; they extend, when fully developed, into the third segment after the ovarian 

 segment ; the sperm-sac comes to lie within the egg-sac. These sacs are not found 

 in Chaetogaster. A further difference from Chaetogaster is in the absence of genital 

 setae. The ventral setae of the ovarian segment are quite absent in fully mature 

 worms. This segment, as well as the one in front, is occupied by the clitellum. 

 The testicular segment contains, in addition to the testes, the spermathecae : these 

 are placed behind the anterior septum of that segment. 



The genus Amphichaeta, although it should clearly be placed in the formerly-used 

 family Chaetogastridae, on account of the peculiar ' cephalization ' and the structure of 

 the alimentary canal, differs from that family, and approaches the Naidomorpha in 

 the presence of sperin-sacs and of egg-sacs; the absence of genital setae removes 

 it from other Chaetogastridae, and brings the genus near to such a genus as Dero, 

 where, as I have shown, there are no genital setae ; as a rule, these setae are 

 present in the Naidomorpha. 



(1) Amphichaeta leydigi, TAUBEB. 

 A. leydigi, TAUBER, Ann. Dan. 1879, p. 76. 



Definition. One seta m first bundle, 2-4 in the rest. Hali. Denmark; Germany. 



(2) Amphichaeta sannio, KALLSTENIUS. 

 A. sannio, KALLSTENIUS, Biol. Foren. Fb'rh. 1892, p. 54. 



Definition. Length, 1-5 in. ; four setae in each of f/iree anterior seta-bundle*, afterwards three. 

 Had. Sweden ; Baltic. 



Genus CHAETOGASTER, VON BAER. 



Syn. Nais, LAMARCK, &c. (in part.). 

 Mutzia, VoctT. 

 Derostoma, DUGES. 



DEFINITION. Ventral seta-bundles only present, composed of uncinate setae. 

 The first in segment II. Segments III-V without setae at all (except in 

 Ch. filiformiz). One pair only of commissural blood-vessels. 



