Dr. Johnston on Scottish Mollusca. 4 J 



gression of our natives is usually slow, even, and continuous. 

 They creep along the bottom or among the branches of coral- 

 lines; but when confined in a basin they ascend to the surface, 

 place themselves in a reversed position, and thus glide along 

 it with ease, aiding themselves by undulations of the cloak and 

 its expansions. Lamarck erroneously asserts that the Bolides 

 and Tritonia cannot swim *, for this seems in fact to be their 

 favourite mode of progression, and in which they exert their 

 locomotive powers with most success. When laid on the sur- 

 face I have seen several of the smaller sorts form the posterior 

 edge of the foot or tail into a kind of circular sucker, and by 

 its means hang pendent for some time. 



The mouth is situated in the front of the nearly acephalous 

 body between the overlapping cloak and the foot : it is a sub- 

 circular or vertical aperture with fleshy lips, which the Doris 

 can protrude to a considerable extent to form a short proboscis. 

 At the sides of the mouth there is usually a pair of fleshy fila- 

 ments, more or less elongated, which appear to perform the 

 office of feelers to guard against the entrance of noisome food, 

 and to select what may be agreeable ; and above it we perceive 

 a development of the cloak with a laciniated margin which has 

 received the denomination of the oral veil. The mouth is either 

 emaxillary, or, as in Tritonia, furnished with a pair of large 

 corneous jaws, which, moved by powerful transverse muscles, 

 serve to divide the sea-weed on which the animals feedf ; the 

 fare of the others being presumed to be of a softer nature and 

 less restricted in kind J. Forced by appropriate muscles down 

 the oral or proboscideous canal, the food is next laid hold on 

 by the tongue, a broad membrane of the most delicate and 

 beautiful mechanism, consisting of a lace-work of minute 

 prickles set in regular array with their sharp points all pointed 

 backwards §. Passing over this membrane the vegetable tex- 



* Anim. s. Vert. vi. i. 301 and 304. 



\ " Ces deux lames sont fort tranchantes, et il n'est rien de vivant qu'elles 

 ne puissent couper lorsque l/animal en fait glisser les deax tranchans l'un 

 sur l'autre." — Cuvier, Mem. sur la Tritonia, p. ] 1. 



\ Risso says that the nourishment of the Nudibranchia consists of small 

 zoophytes. — Hist. Nat. de l'Europ. Merid. iv. 40 ; see also Loudon's Mag. 

 Nat. Hist. viii. 78. 



§ Dr. Fleming says that the tongue of Tritonia " differs remarkably from 

 the same member in the Doris. In the latter, the spines with which it is 



