MOLLUSCA 



5473 



MOLLUSCA 



in tin- chitons or " coat-of mail 

 ..hi-IN ": < ..in r.ilnl liy the mantle in 

 the slug* ; or quite internal, as the 

 M. call. -,l 'bone" of the cuttlee. 

 They nmy be smooth and highly 

 1 1 on the exterior, or covered 

 Mithiiilijcs.knobs.andspines. Every 

 kin n tint of colour occurs in their 

 cm. i mentation. The aperture may 

 be a simple opening, or so obstruc- 

 ted l.y folds or "teeth" that it 

 seems marvellous that the animal 

 can get out. The mother-of-pearl 

 -. i nerve consists of the inner 

 lining of certain shells ; while 

 pearls consist of the same material, 

 usually secreted around an intru- 

 >i\e pt'rasite or a grain of sand, to 

 protect the tissues from irritation. 



Molluscs are widely distributed. 

 The majority are marine, but many 

 inhabit fresh water, and others are 

 terrestrial. They range from the 

 tropics to the Arctic regions ; and in 

 the ocean they occur from the 

 shallows by the shore to depths of 

 2,000 fathoms. Though moisture is 

 essential to their existence, some 

 species manage to live in hot 

 deserts, where they support life on 

 the barest scrub, and are only 

 refreshed by rain at very rare 

 intervals. It has been estimated 

 that there are over 50,000 living 

 species, of which the gastropods 

 account for over 40,000 and the 

 bivalves for about 9,000. But the 

 widely differing views of systema- 

 tists as to what constitutes a 

 species make such estimates un- 

 certain and provisional. 

 Circulatory System 



Most of the molluscs have a well- 

 developed circulatory system, cen- 

 tring in a heart which may have 

 one or two ventricles and auricles. 

 Arteries and veins convey the 

 blood from and to the heart, but 

 it is doubtful if any true capillary 

 system exists, except perhaps in a 

 very few species. Usually the 

 blood is conveyed to lacunae or 

 spaces between the internal organs. 

 It is aerated in the case of marine 

 species by gills which extract 

 oxygen from the water, and these 

 gills are usually within the mantle 

 cavity. In the case of the nudi- 

 branchs they are exposed on the 

 exterior of the body. In the pulmo- 

 nates, which include the terres- 

 trial species generally and most of 

 the fresh-water snails, no gills are 

 present, but the interior of the 

 mantle acts the part of a lung, its 

 inner surface being covered with 

 finely branching vessels, which 

 bring the blood into close contact 

 with the air. 



The alimentary canal com- 

 mences with a mouth which, except 

 in the bivalves, is provided with a 

 radula or lingual ribbon. This con- 

 sists of a strip of chitin, covered 



with numerous rows of teeth, which 

 moves backwards and forwards 

 over a pad. By this means the food 

 is rasped into fragments before be- 

 ing swallowed and conveyed to the 

 crop, where it is acted upon by the 

 secretions of the liver and other 

 glands. In the cephalopoda there 

 is also a horny beak, much like that 

 of the parrot. In the gastropoda the 

 alimentary canal with its attendant 

 viscera undergoes torsion and is 

 more or less twisted into a figure of 

 eight, thus bringing the anus round 

 to one side of the back and for- 

 wards ; while in the cephalopoda it 

 is shaped like a letter U, and the 

 anus is near the mouth, but separ- 

 ated from it by the " arms." 



Most molluscs appear to be pro- 

 vided with the usual five senses. 

 They are all very sensitive to touch, 

 notably along the edge of the man- 

 tle, which is often specialised into 

 lobes that appear to have a tactile 

 purpose. The tentacles on the head 

 also serve the same purpose, and 

 the foot is extremely sensitive. 

 Most of the species, except the bi- 

 valves, have eyes on their head, 

 either upon or closely associated 

 with the tentacles or horns. In the 

 cephalopods the eyes are very com- 

 pletely developed ; while in many 

 bivalves they are represented by 

 sensitive ocelli arranged along the 

 edges of the mantle. But it is very 

 doubtful if any mollusc, except the 

 cephalopods, can see much. The 

 sense of smell is well developed, 

 though very little is yet known 

 about their olfactory organs. 

 Characteristics of Species 



Reproduction in the molluscs is 

 sexual ; in some species the sexes 

 are in separate individuals, while 

 in others hermaphroditism occurs. 

 But the action of two individuals 

 is always needed, self-fertilisa- 

 tion appearing to be a very rare 

 and abnormal occurrence. The 

 young are always produced from 

 eggs, which are usually hatched ex- 

 terior to the body of the parent ; 

 but in the bivalves and some gas- 

 tropods they are retained within 

 the body or the shell of the parent 

 till after hatching. The common 

 swan mussel of British ponds is 

 estimated to produce over a million 

 eggs, and the same is true of the 

 edible oyster ; while the common 

 dog whelk, which swarms on the 

 rocks around Gt. Britain, has been 

 observed to produce over 120,000 

 eggs. The land and fresh-water 

 species are not so prolific, the com- 

 mon snail (Helix aapersa) laying 

 from 40 to 100 eggs each summer. 

 Certain S. American snails lay eggs 

 as large as pigeons' eggs. 



Molluscs are herbivorous, carni- 

 vorous, and omnivorous. The ma- 

 jority of the land species feed upon 



vegetable*, but many land and 

 fresh-water species are more or less 

 carnivorous. Among the marine 

 specie*, nearly all those whose 

 shells are round-mouthed and have 

 no notch or channel at the inner 

 lower corner are vegetarian and 

 feed upon growths on the rocks, as 

 the limpets. Where the notch 

 exists, e.g. with the whelks, it may 

 usually be safely assumed that the 

 animal is carnivorous. These feed 

 upon carrion and other shellfish. 



The length of life in the Mollusca 

 varies greatly. Mussels are full- 

 grown in a year and oysters in five 

 years, though they have been 

 known to live ten. The swan 

 mussel is believed to live from 20 

 to 30 years, and the common peri- 

 winkle has lived in an aquarium for 

 20 years. 



Principles ol Classification 



The classification of the Mollusca 

 is still a matter of difference of 

 opinion among malacologists. The 

 following arrangement covers the 

 main points and has at least the 

 merit of simplicity. It may be 

 amplified by reference to the works 

 mentioned below. 



Class I. GASTROPODA (stomach- 

 footed). These have a flat foot 

 adapted for crawling. 



Div. 1. Amphineura. In these 

 the internal organs are bi-lateral 

 and symmetrical ; and in most the 

 shell consists of eight plates ar- 

 ranged along the back. Example, 

 the chitons. 



Div. 2. Streptoneura. In these 

 the visceral organs have been 

 twisted into a figure of eight loop ; 

 the gill is in front of the heart ; and 

 the sexes are separate. Examples, 

 limpets, whelks, and periwinkles. 



Div. 3. Euthyneura. In these the 

 visceral loop has become untwisted; 

 the gill if any is behind the 

 heart ; and the animals are herma- 

 phrodite. Examples, sea slugs, and 

 land and fresh-water snails. 



Class II. SCAPHOPODA (digging- 

 footed). These have a nearly 

 straight, tubular shell ; and the 

 foot is long and cylindrical. Exam- 

 ples, the elephant tusk shells. 



Class III. PELECYPODA (axe- 

 footed). This class includes all the 

 bivalves. Examples, the oyster 

 and mussel. 



Class IV. CEPHALOPODA (head- 

 footed). In these the front part of 

 the foot is developed into a series of 

 sucker-bearing arms about the 

 head. Examples, the cuttles, 

 octopus, and nautilus. 



Economically, the Mollusca are of 

 great importance to man. Nearly all 

 of them are edible, though it must 

 be admitted that most of them are 

 not attractive. They are also much 

 used as bait in fishing. The shells 

 are used for ornamental purposes 



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