XII PHYLUM MOLLUSOA 721 



in mud, others live in association with various colonial 

 Coelenterates. The Placophora are all vegetable feeders, their food 

 consisting of minute algae and diatoms. 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 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 the 

 one-sided development of the viscei'al 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 successiveh' 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. 



1. Example of the Class. — The Triton (Triton nodiferus). 



Triton is a marine Gastropod living in shallow water, usually 

 close inshore. The species to which the following desci'iption 

 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 {Bucdnum un- 

 datum) will be found to conform to the description. 



The shell (Fig. 608) is a very hard and dense calcareous 

 structure, presenting no trace of division into valves such as 

 compose the shell of the freah-water Mussel, and lacking also 

 its bilateral symmetry. It is in the form of an elongated hollow 

 cone closely wound round a central axis. The apex of the cone 



