NORWEGIAN OPISTHOBRANCHIATE MOLLUSCA 37 



of the rhinophores of the Opisthobranchia, that their sheaths are 

 a remnant of the primitive conditions prevaihng in some Tecti- 

 branchia, and that this persistence thus marks a less advanced 

 stage of development compared to forms without rhinophorial 

 sheaths. Consequently, the Dotonidae may represent a more pri- 

 mitive type than the Aeolididae, where sheaths are absent. This 

 opinion is in contrast to the general ideas of the relations of 

 Dotonidae, expressed by Pelseneer, that these are derived 

 from the Aeolids. In reality other primitive features in Dotoni- 

 dae seem to support the new opinion, for instance the absence of the 

 cnidosacs of the aeolidian type (small isolated organs of homologous 

 structure occur, hovs^ever, cf. Vayssière, 1888); the occurrence 

 of papillär cells of the same nature as in Hermaeidae (cf. 

 Pelseneer 1895); the somewhat ascoglossan type of the radula 

 (cf. Vayssière 1888). In order to base a judgment of the affi- 

 nity on a more comprehensive mass of facts, I here give some 

 further statements on the anatomy together with some new 

 points of view for the solution of this problem. 



The Digestive System. 



In its essential features this is well described, already, by 

 Alder & Hancock 1845, who also notice the great difference 

 in respect to the Aeolididae, that in Doto the great posterior 

 livôr duct lies beneath the hermaphrodite gland, whereas its 

 position is the reversed in the Aeolididae. A further discrepancy 

 is offered by the rectum, which in Aeolidia passes on to the 

 right side beneath the liver ducts which extend into the fore- 

 most rows of papillae; in Doto there is no such relation, 

 because the rectum is very short, and leads directly to the 

 dorsal anus behind the first papilla; from this, a duct debouches 

 directly into the stomach. To the descriptions of the intesti- 

 nal canal given by Alder & Hancock, Bergh and Vayssière, 

 sorue facts may be added concerning the bulbus pharyngeus. 



