CASTKKol'ODA 



111 



median, ami -ule-like. It i- tin- surface on which 

 tin- aiiiiiial crawls, ami is often divided into 

 anterior, median, ami posterior regions. The 

 \\c-.-ihli i.i" modification indinled in the ol*M is HO 

 great that no other jjnirral characters can bo 

 given. 



General Survey. (A) The simplest Gasteropods, 

 such as the common Chiton, are symmetrical, not 



Fig. 1. Part of the Rasper of the Snail (from Howes). 



lop-sided like the higher forms. They have the 

 mouth at one end of the long axis of the body, the 

 anus at the other ; the gills, kidneys, genital 

 ducts, and circulatory organs are paired ; there 

 are two pail's (pedal and visceral) of nerve cords 

 running parallel to one another along the body, 

 ami the ganglia are slightly developed. Of all 

 molluscs these simplest Gasteropoda are probably 

 nearest the hypothetical worm-like ancestor. In 

 one order (Chiton, q.v.) there are eight shells, one 

 behind, the other like segments ; in the two other 

 orders (Neomenine and Chsetoderma) the shell is 

 represented only by calcareous plates and spines in 

 the skin. These three orders form the sub-class 

 Isopletira, in contrast to all the others which are 

 unsymmetrical the Anisopleura. 



(fe) The latter are grouped first of all according 

 to the state of the loop formed by the visceral 

 nerves. ( 1 ) In one series the visceral nerve-loop is 

 implicated and twisted in the torsion of the asym- 

 metrical body, 

 and furthermore 

 the sexes are 

 separate. These 

 are known as 

 Streptoneura 

 (' loop -nerved'), 

 and include 

 limpets (Patella), 

 ear-shells ( Hali- 

 otis), pond-snail 

 (Paludma), cow- 

 ries (Cypnea), 

 cone-shells 

 (Conns), buckies 



Fig. 2. A Whelk: 



Showing respiratory siphon, a; head with } R 

 tenacles, c, and eyes, d; foot, ft, with ( Buccinum), and 

 shell-lid or operculum. t. the free-swim- 



ming Heteropods. 



This division includes what are often called Proso- 

 branchs, and the numerous genera are further 

 arranged according to the characters of the gills, 

 kidneys, and foot. (2) In another series the vis- 

 ceral loop is not twisted, and is often very short ; 

 the shell is light and often lost in the adult ; and 

 the animals are hermaphrodite. They are known 

 as Euthyneura ( 'straight-nerved'), and include two 

 sets Opisthobranchs and Pulmonates. Among 

 Opisthobranehs some retain the usual mantle-fold 

 anil have a delicate shell e.g. Bulla and Aplysia, 

 while others (known as Nudibranohs ) have their 

 mantle atrophied and no shell e.g. Doris and Eolis. 

 Lastly there are the Pulmonates, where gills are 

 replaced by an air-breathing mantle-cavity, as in 

 snails (e.g. Helix), slugs' (e.g. Arion), water-snails 

 (e.g. Lymnneus). 



Mode of Life. Though the number of terrestrial 

 Gasteropoda, breathing the air directly by means 

 of a pulmonary chamber, is very large over 6000 



living HjH-cies ih.e living in water are greatly in 

 the majority, including over 10,000 form-, mostly 

 marine. Of* these, some !M)00 or so ln;long to the 

 ProsohrancliH or Streptoneura, a relatively small 

 minority Iming Opistnobranchs and Nndibranch*. 

 The Heteronods and Home OpisthohrancliH enjoy a 

 free-Hwimming pelagic life, but most marine form* 

 treijiii-ni the coast** either on the shores or along 

 the bottom. Deep-sea GaHteropods are compara- 

 tively few. The locomotion effected by the con- 

 tractions of the muscular ' foot ' is in almost all 

 cases very leisurely, 

 and the average tend- 

 ency is towards slug- 

 gishness. As to diet, 

 the greatest variety 

 obtains ; most Proso- 

 branchs with a re- 

 spiratory siphon and 

 a corresponding notch 

 in the shell are car- 

 nivorous, and so are 

 the active Hetero- Fig. 3. 



pods; most of the Young Pond Snail (Lymnceug) 

 rest are vegetarian (from Howes), 



in diet. Numerous 



genera, both marine and terrestrial, are very indis- 

 criminate in their feeding ; others are as markedly 

 specialists, keeping almost exclusively to -some 

 one vegetable or animal diet. Some marine 

 snails partial to Echinoderms have got over the 

 digestive difficulty presented by the calcareous 

 character of the skins of their victims by a secre- 

 tion of free sulphuric acid from the mouth. This 

 acid changes the carbonate of lime into sulphate, 

 which is brittle and readily pulverised by the rasp- 

 ing tongue. A few are parasitic e.g. Eulima, 

 Stylifer, and the very degenerate Entoconcha 

 mirabilis, all occurring in or on Holothurians. 



Distribution. A few Gasteropods occur in strata 

 as far back as the Cambrian, from which remote 

 period they have continued with a steady increase. 

 Almost all the Palaeozoic genera are no\v extinct, 

 and during these ages the siphon-possessing forms 

 seem to have been almost, if not altogether, unre- 

 presented. A host of new Gasteropods appeared 

 in the Jurassic period, and many of the modern 

 families have their origin in Cretaceous times. 

 Numerous as the fossil forms 

 are, the number of types wholly 

 extinct is comparatively small ; 

 both as regards persistence of 

 types and increase of numbers, 

 the Gasteropods are a peculiarly 

 successful class. 



Life-history. The eggs of 

 Gasteropods are usually small, 

 and are surrounded with albu- 

 men, the surface of which be- 

 comes firm, while in the com- 

 mon snail (Helix) and some 

 others there is an egg-shell of 

 lime. The eggs not unfre- 

 quently develop into embryos 

 within the parent, but in most 

 cases they are laid, either singly 

 or in masses, and often within 

 cocoons. Few objects are more 



Fig. 4. 



familiar on the seashore than Section of Triton-shell 

 the clustered egg-cases of the (after Owen 

 whelk which together form a "S 

 hall often about the size of 

 an orange. Inside each of the numerous egg 

 are many embryos, but only a few reach maturity, 

 the others serving as fowl material, an infant ile 

 cannibalism or struggle for existence not uncom- 

 mon in the class. As to the actual develop- 

 ment and the larval forms, reference must be made 



