CLASSIFICATION. 189 



CLASSIFICATION. 



Judging from the material already descriljod, it is evident that not only 

 is the genus Chaetoderma very widely distributed, but the individual species 

 are difficult to differentiate, especially where we are compelled to rely solely 

 upon descriptions and figures. Externally and internally there is considerable 

 variation within a given species, and suitable diagnostic characters are difficult 

 to discover. The relations of the gut to the somatic musculature and the ventral 

 blood sinus at the junction of the pro- and metathorax are of considerable value 

 in the specimens I have thus far examined, and with figures of this region and 

 others of characteristic sections it is believed that it wiU be possible, without 

 great difficulty, to recognize the species described in this and the earlier Pacific 

 Ocean report. 



The validity of the genus Dorymenia Heath has been questioned by certain 

 investigators, but the discovery of a second species, with all of the character- 

 istic features of the first {Dorymenia acuta), indicates that the genus was well 

 founded. 



Order Aplacophoea v. Ihering. 



Suborder I. Chaetodermatina Simroth. 



Spiculose integument continuous all around the body. 

 Chaetodermatidae, p. 189. 

 Suborder II. Neomeniina Simroth. 



Spiculose integument interrupted beneath by a longitudinal ven- 

 tral furrow. 

 Neomeniidae, p. 191. 

 Proneomeniidae, p. 191. 

 Dondersiidae, p. 192. 



CHAETODERMATIDAE Simroth. 



Opening of mouth and anal chamber terminal. Body with more or less 

 sharply marked regions. Ventral furrow and fold lacking. Two highly devel- 

 oped plume-like gills. Radula distichous, polyserial or strongly reduced to a 

 large unpaired cuticular tooth. The mid-gut possesses, in most cases, a well- 



