NORWEGIAN OPISTHOBRANCHIATE MOLLUSCA 43 



this arrangement thus represents the more primitive stage, and 

 forms with this arrangement preserved may be considered as 

 retaining a more primitive organization. 



The different mutual position of liver and genital organs, 

 which is to be observed among the Cladohepatic Nudibranchia, 

 can easily be explained by the following supposition. Primarily, 

 the hermaphrodite gland originates in the median part of the 

 body. As the left liver lobe increases considerably, the genital 

 organs get a position to the right of the chief mass of the liver; 

 this position it keeps in primitive forms such as Goniaeolis. 

 By a greater expansion of the liver, this either 1) locates beneath 

 the hermaphrodite gland (as a rule): the primitive conditions 

 may still be indicated by the course of the large liver canal 

 chiefly on the left side of the body, (e. g. Pleurophyllidia, 

 Madrella), or a secondary replacement results in a median 

 position of the same (e. g. Tritonia) ; 2) gets a position dorsal 

 of the hermaphrodite gland (Aeolididae). 



In the Ascoglossa (Elysioidea) the development has followed 

 a different direction from the very beginning, inasmuch as the 

 right liver lobe has been enlarged so as to spread, finally, over 

 the whole right side of the body, while the left liver portion is 

 restricted to the left side; from each a special duct leads towards 

 the stomach. As Pleseneer (1894) remarks this symmetrical 

 arrangement is a secondary acquisition since on the larvel stage 

 the left liver lobe also in these forms is smaller than the right 

 one. A stage somewhat intermediary between this and that of 

 the Aeolididae is represented by Antiopa (cf. Alder & Hancock). 



As a result of the above argumentation we must reject 

 Pelseneer's views about the Dotonidae as d from Aeolidian 



ancestors, and consider them as a side-branch from an earlier 

 stock of the Cladohepatic Nudibranchia, a stock with primitve 

 characters and a potency of developing in several independent 

 directions. One of these has resulted in the recent Aeolididae, 



