NORWEGIAN OPISTHOBRANCHIATE MOLLUSCA 41 



In its distal portion the vas deferens (v. d.) forms some 

 windings and enters into the penis [p), which it pierces as far 

 as the pointed end; the penis is unarmed, and included in a 

 sheath, or cavity, in the wall of the body. 



Sensory Organs. 



The eyes have an organization as simple as usual among 

 the Nudibranchia, containing only a lens and a surrounding cup 

 of pigmented cells. They lie beneath the body epithelium at 

 the side of the rhinophorial ganglia, from which they are sepa- 

 rated being innerved by distinct optic nerves from the cerebral 

 ganglia. 



The rhinophores contain a large ganglion in their roots 

 which sends a nerve through their central pillars. These are 

 clothed with a densely ciliated sensory epithelium, while the 

 sheaths consist of a double layer of epithelium, the outer one 

 passing into that of the body surface; the inner one, being 

 thickest beneath the upper margins of the sheaths, thins out 

 considerably towards their base and passes into the sensory 

 epihelium of the central pillar. 



General Remarks on the Relations of Dotonidae. 



Taken as a whole the organization of Doto indicates that 

 this genus has early separated from the stock of Cladohepatica 

 which embraced the most ancient types. Doto i. a. still keeps 

 the rhinophorial sheaths which are reduced in the true Aeoli- 

 doidea. This fact offers a proof against Pelseneer's opinion of 

 the derivation of the Dotonidae from the Aeolidoidea. If namely 

 the last-named group has originated, as is the meaning of 

 Pelseneer, from forms such as Bornella (with rhinophorial 

 sheaths), and has secondarily lost the same, it is improbable 

 that from such forms new types with sheaths may have origin- 



