42 NILS HJ, ODHNER 



ated. Pelseneer himself refers to this law of irreversibility of 

 evolution in other cases. 



Also the comparatively simple habitus of the nephridium with 

 its slight lobation indicates conditions which are only met within 

 primitive types of Cladohepatica, and separates Dotonidae from 

 Aeolidoidea, where the nephridium is greatly ramified. In its 

 short pericardial tube of the kidney Doto, however, differs from 

 such primitive forms as Tritonia (cf. Pelseneer 1894) or 

 Goniaeolis (cf. above). 



As to the intestinal canal, it offers, as already mentioned, this 

 difference from the Aeolidoidea, that the liver ducts run on the 

 ventral side of the hermaphrodite gland. In the mutual position 

 of these viscera there prevail some discrepancies among the 

 Nudibranchia, inasmuch as the more primitive Cladohepatica, 

 (with a relatively compact liver) generally have the chief liver 

 duct either on the upper median side of the hermaphrodite gland 

 (e. g. Tritonia, Dendronotus), or on the left side of it {Linguella, 

 Pleurophyllidia, Pleuroleura, Antiopa, Madrella; cf. Bergh, 

 Malac. Unters,; and Odhner 1917^; Goniaeolis). The organization 

 of these forms makes it evident that this liver duct arises from 

 the primary left liver lobe; in advanced forms, such as Aeolidia, 

 it has enlarged considerably, and spread even to the right side 

 of the body. The primary right hver lobe is represented in all 

 Cladohepatica by the portion of the liver which is situated to 

 the right of the stomach and in front of the intestine and which 

 opens by means of the right canal or canals directly into the 

 stomach. In the Holohepatic Nudibranchia this part of the liver 

 has been transferred into the gall bladder, an organ which is 

 entirely lacking in all Cladohepatica, but present, with a few 

 exceptions, in all Holohepatica. In the latter group (except Tre- 

 velyana), the liver always lies beneath the hermaphrodite gland; 



1 In fig. 21 of my worli of 1917 (K. Sv. Vet. Akad. Handl. Bd. 52) 

 this right duct has been omitted; a revision has proved its existence. 



