CASTRO-ENTERITIS 



3444 



GASTROPODA 



10 



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IS 



Gastropoda. 1. Rounded snail, Helix rotundata. 2. Beautiful snail, H. 

 pulchella. 3. Rock snail, H. rupestris. 4. Prickly snail, H. aculeata. 5. Cheese 

 snail, H. obvaluta. 6. Lapidary snail, H. lapicida. 7. Plated snail, H. lamel- 

 lata. 8. Bristly snail, H. hispidus. 9. Ruddy snail, H. rufescens. 10. Silky 

 snail, H. granulata. 11. Green hairy snail, H. revelata. 12. Dusky snail, H. 

 fusca. 13. Kentish snail, H. cantiana. 14. Sandhill snail, H. pisana. 15. Copse 

 snail, H. arbustorum. 16. Carthusian snail, H. carthusiana 



the end of the femur and the other 

 from the inner side. These heads 

 of the muscle gradually meet as 

 they pass down the leg, and ter- 

 minate in a broad tendinous band 

 which is continued as a strong 

 tendon, the tendo Achillis. See 

 Tendon of Achilles. 



Gastro- enteritis. Inflamma- 

 tion of the stomach and intestines. 

 See Enteritis. 



Gastropoda( Gr. gasier, stomach; 

 stem pod, foot). One of the great 

 divisions of the sub-kingdom Mol- 

 lusca. It includes those molluscs 

 which have the ventral or under side 

 of the body developed in a gliding 

 base. Gastropods may be roughly 

 defined as comprising snails and 

 slugs, terrestrial, fresh -water, and 

 marine. The whelk is a familiar 



example of a marine gastropod. 

 The underside of a gastropod is 

 its organ of locomotion, and its 

 mode of action may be seen by 

 watching a land snail crawl on the 

 window-pane, or a pond snail on 

 the glass front of an aquarium. 

 The body is slowly propelled 

 forwards by a peculiar ripple or 

 wave -like movement of the foot. 



The body, which lies above the 

 foot, consists of a well-defined 

 head and a visceral hump, covered 

 by an outer glandular layer known 

 as the mantle. In slugs this hump 

 is not very noticeable, but in snails 

 it is long and coiled, and protected 

 by a shell usually more or less 

 conical in form. This visceral 

 hump contains most of the internal 

 organs. During development the 



internal organs of a gastropod 

 undergo a kind of torsion or twist- 

 ing, the result of which is to bring 

 the posterior termination of the 

 alimentary canal towards the head 

 and the left-hand organs to the 

 right. The nerve loop which in- 

 volves the visceral organs thus 

 becomes twisted into a figure of 

 eight. ' The original left-hand 

 organs, now on the right side, 

 become atrophied, the result being 

 that the internal arrangements of 

 a gastropod are not symmetrical, 

 and there is only one kidney, one 

 gill, and one auricle to the heart. 



Another interesting feature is 

 the odontophore or lingual ribbon, 

 situated at the back of the mouth. 

 This is a long, horny band, called 

 the radula, studded with a vast 

 number of minute teeth. When a 

 gastropod is feeding the substance 

 is seized by the jaw, and the radula 

 moves backwards and forwards 

 like a rasp, and scrapes off minute 

 particles which pass into the 

 stomach. 



Gastropods are divided into two 

 sub-divisions : the Streptoneura, 

 in which the twisting is well 

 marked, and the Euthyneura, in 

 which the visceral hump appears 

 to be partly untwisted. The first 

 sub-division contains two orders : 

 the Aspidobranchia, which have 

 flattened leaf-like gill filaments, 

 and the Pectinibranchia, in which 

 the gill filaments are elongated. 

 The second sub-division is also sub- 

 divided into two orders : the 

 Opisthobranchia, in which the 

 heart is placed in front of the 

 gills, and the Pulmonata, in which 

 the gills are absent and the mantle 

 cavity serves as a kind of lung. 

 Most land and fresh-water gastro- 

 pods belong to this order. 



Economically, certain marine 

 gastropods are of value as food for 

 man, as the whelk and periwinkle ; 

 and several species of land snails 

 are consumed on the Continent, 

 and to a small extent in Great 

 Britain. Some aquatic gastropods 

 do useful work as scavengers, while 

 many of the terrestrial ones do 

 great mischief in gardens by eating 

 the plants. (See Snail.) 



Fossilised remains are found 

 throughout the stratified rocks. 

 Air-breathing forms were first met 

 with in Devonian rocks, while 

 fresh -water snails were first found 

 in Purbeck rocks, of much later 

 age. See Mollusca. 



Bibliography. Proceedings of the 

 Malacological Society of London, 

 ed. B. B. Woodward, 1893, etc. ; 

 Monograph of the Land and Fresh- 

 water Mollusca of the British Isles, 

 John William Taylor, 1894, etc. ; 

 Shell Life, E. Step, 1901 ; The Life of 

 the Mollusca, B. B. Woodward. 1913. 



