702 ZOOLOGY SECT. 



on the shore between tidal limits. The Aplacophora, on the 

 other hand, are rare in very shallow water, and absent altogether 

 from the littoral zone : some have been found at considerable 

 depths (down to 1,250 fathoms). Some of them burrow in mud, 

 others live in association with various colonial Coelenterates. 

 The Placophora are all vegetable feeders, their food consisting of 

 minute algae and diatoms. When at rest, they adhere firmly 

 to the surface of a rock or a block of coral by means of the 

 sucker-like foot. When forcibly detached the animal curls itself 

 up into a ball, and will only after a considerable time slowly extend 

 itself again : all its movements are extremely sluggish. 



The Aplacophora have no hard parts that would be recognis- 

 able in the fossil condition ; but numerous fossil Placophora are 

 known from the Silurian onwards. The valves of the Silurian 

 genera differ from those of recent forms in the absence of the 

 articulations. 



CLASS III. GASTROPODA. 



The Gastropoda, including the Snails and Slugs, Limpets, 

 Whelks, Periwinkles, Sea-hares, and the like, are Mollusca in 

 which there is, as a rule, a shell composed of a single piece, 

 and in which the mantle is not divided into two lateral folds as 

 in the Pelecypoda. The body is inequilateral, owing to 

 one-sided development. There is a well-developed ventral foot, 

 usually with a broad, fiat surface on which the animal creeps. 

 A head-region bearing eyes and tentacles is distinguishable in 

 front of the foot. The alimentary canal is characterised by the 

 presence in the buccal region of a peculiar organ, the odontophore, 

 present also in some of the Amphineura, bearing rows of minute 

 chitinous teeth. Plume-like ctenidia are usually present. A 

 metamorphosis occurs in the development, during which the young 

 Gastropod passes successively through trochophore and veliger 

 stages. The majority of the families of Gastropoda are marine, 

 a few of these being pelagic ; but some inhabit fresh water, and 

 others are terrestrial. 



]. EXAMPLE or THE CLASS. THE TRITON (Triton rubicundus). 1 



Triton is a marine Gastropod living in shallow water, usually 

 close inshore. The species to which the following description 

 specially applies has a very wide range, from the English Channel 

 to the South Pacific, and occurs as a fossil as far back as the Miocene. 

 In most respects the English Whelk (Buccinum undatmn) will be 

 found to conform to the description. 



The shell (Fig. 618) is a very hard and dense calcareous structure, 

 presenting no trace of division into valves such as compose the 



1 Or Gharonia rubicunda. 



