xii PHYLUM MOLLUSCA 671 



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

 other hand, are rare in very shallow water, and absent alto- 

 gether from the littoral zone ; some have been found at con- 

 siderable depths (down to 1,250 fathoms). The Placophora are 

 all vegetable feeders, their food consisting of minute alga? 

 and diatoms. The Aplacophora subsist on small animals. 

 The Placophora when at rest 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 their movements are extremely sluggish. The Aplacophora 

 are unable to fix themselves in this way ; many of them occur 

 twined round the stems of zoophytes, sometimes attached by a 

 thread of viscid mucus. 



The Aplacophora have no hard parts that would be recognisable 

 in the fossil condition ; but numerous fossil Placophora are known 

 from Silurian formations 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 the 

 one-sided development of the visceral mass. There is a well- 

 developed ventral foot, usually with a broad flat 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 

 Trochosphere 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. 



1. EXAMPLE OF THE CLASS. THE TRITON (Triton nodi/ems). 



Triton is a marine Gastropod living in shallow water, usually 

 close in shore. 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 un- 

 datum) will be found to conform to the description. 



