246 



SHELLS AND SHELL-FISH. 



testaceous part of the animal consists of a row of sym- 

 metrical scales^ generally imbricate, or reposing on each 

 other transversely along the middle of the back ; but 

 sometimes_, as in Chitonellus, these valves are widely 

 separated. The only naturalist who has effectually in- 

 vestigated these shell-fish, is our late regretted friend 

 Guilding ; and to his admirable paper upon them, we 

 are indebted both for their natural arrangement, and the 

 following notes on their manners when living : — "These 

 animals frequent the rocks and stones of the sea coast, 

 and are distributed nearly over the whole globe. Many 

 species are constantly under water ; while others ascend 

 even above high water mark, spending the day exposed 

 to the hottest sun, or resting in spots occasionally 

 moistened by the rude and restless surf. In Chitonellus 

 and Cryptoconchus, there are certain minute organs on 

 the zone, which bear a strong resemblance to the spi- 

 racula of annulose animals. They seem to feed en- 

 tirely by night. Though they remain stationary during 

 the day, yet when disturbed they will often creep away 

 with a slow and equal pace ; sometimes sliding sideways, 

 and creeping under rocks or stones for concealment. 

 If accidentally reversed, they soon recover their posi- 

 tion by violently contorting and undulating ; and for 

 defence, they sometimes roll themselves up, like the 

 woodlice."* Although only two or three small species 

 are found in Europe, there are numerous large ones 

 from the tropical latitudes, particularly the coasts of 

 Chili and Peru. 



(230.) The Tectibranchia constitute the last group 

 in our arrangement of the Gasteropoda. In it we in- 

 clude all those naked testaceous moUusks, whose shell, 

 when it exists, is either rudimentary or partly con- 

 cealed by the lobes of the mantle; and whose branchia, 

 in the form of leaves, are arranged either on one or 

 both sides of the body, or on the back, but in all cases 

 concealed or covered either by the edge of the mantle 

 or by a thin shell. We do not think that these animals 

 * Zoological Journal, v. 29. 



