SUB-KINGDOM III.— MOLLUSCA. 



SNAILS, ETC. 



By B. B. Woodward, F.L.S., F.G.S., Etc. 



The sub-kingdom Mollusca includes such well-known creatures as the whelk, 

 the oyster, the garden snail, and the octopus. It may be roughly character- 

 ised as comprising soft-bodied animals that are usually possessed of a hard 

 shell into which they can withdraw for protection. The forms of its dififerent 

 members are as varied as the different conditions of life to which they have 

 become adapted ; for some live in the open sea, others hide in its depths ; 

 some live in rivers or swift-flowing streams and even torrents, or abide in 

 still lakes and ponds ; others, again, are adapted for life on dry land, and, 

 though mostly lovers of moist, shady spots, will some of them rejoice in 

 sun-baked rocks, or live in arid spots where vegetation is scanty. Indeed, 

 save for the frozen polar regions and snow-capped mountain summits, or 

 desert wastes — such as the Sahara — there is no spot on the surface of the 

 globe, nor, unless it be certain of the extreme depths, any point on the sea 

 bottom, without its representative mollusc. But it is in the tropical regions 

 and oceans that they most abound. Their mode of life is as varied as their 

 habitat : some are strong swimmers, others only float in the water ; some 

 crawl on land or under water, others burrow in the mud or sand, or even 

 into hard rocks, whilst a few live in the tissues of other animals ; others, 

 again, attach themselves to different objects more or less permanently ; this 

 is especially the case with the shore dwellers, who have to withstand the 

 action of currents, the sweep to and fro of the tides, or the violence of 

 storm-tossed waters. Their food, too, differs, some being carnivorous and 

 even cannibals, others strict vegetarians, whilst many prefer a mixed diet. 

 Those that burrow, or are attached, generally feed on microscopic organisms, 

 which they obtain from the mud or the water ; whilst the parasites, as all 

 the world over, dine at the expense of their hosts. 



Owing to the great variations in form, and even in structure, which 

 conformity to these widely difierent requirements of life has brought 

 about, it is impossible to draw up any satisfactory definition of a mollusc 

 which shall be fairly applicable to even the majority of the larger groups. 

 Nevertheless, certain characters they do possess more or less in common. 

 The shell, though not universally present, is on the whole, perhaps, the 

 most conspicuous feature, although not the most important part of 

 the animal. They have also a well- developed muscular system, of which 

 that portion pertaining to the foot, or organ of locomotion, is the most 

 prominent. There is a distinct nervous system ; a heart and circulatory 

 system with its associated organs of respiration ; an alimentary system, or 



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