LECTURES ON MOLLUSCA. 223 



diversity of structure. Between the habits of the amphibious Peri- 

 winkles, which crawl half a mile from shore, and the Marine Snails 

 which are always picked up with sea shells ; or between those of the 

 freshwater snails and freshwater Periwinkles, which are found entan- 

 gled in the same group of confervas ; there need not exist any essential 

 difference. The animals of the true Pulmonates however are formed 

 on a lower type from those of the ordinary Sea-crawlers. The senses 

 are less acute ; and the individuals perform the functions both of male 

 and female to each other. The breathing cavity, instead of being open, 

 as in the air and Water-breathing Prosobranchs, is a chamber lined 

 with minute blood-vessels, and open only at a small hole. This is 

 closed by a valve, to shut out the water in the aquatic tribes, and the 

 hot dry air of summer da} r s in the land species. The shape and way 

 of crawling of the snails is too well known to need description. They 

 are all fond of moisture, and more or less slimy. In the extremes of 

 heat, cold, and .drought, they shut themselves up in corners or under 

 ground, and often make a false operculum, pierced with a minute 

 breathing hole, which is thrown off when the genial season begins. 

 In damp mornings and evenings they are in their glory, munching 

 the luxurious vegetation, and leaving their slimy track behind them 

 as they crawl. They were esteemed a great delicacy by Koman epi- 

 cures ; and are still extensively eaten, both in Europe and South 

 America. The young snails do not undergo any transformation, like 

 that of the pteropodous infants of the Sea-crawlers ; their diffusion 

 being sufficiently provided for by ordinary locomotion. Snails are 

 found everywhere, from the Arctic regions to the equator, but are rare 

 in dry and silicious districts, plentiful wherever there is lime and 

 moisture. The continental species are diffused over very wide areas ; 

 but the islands of the tropical seas have each their own peculiar forms, 

 even if very near to each other, or to the main land. Supposing a 

 traveler brought back the snails from a West Indian island, an expe- 

 rienced conchologist could tell at once where they were collected ; but 

 it would be almost impossible to tell the same from the vast expanse 

 of the various United States. 



Snail shells are always lighter than sea shells, having to be carried 

 on the back of the animal without the watery support. Their con- 

 struction is much simpler, abounding in animal matter ; and they are 

 first formed, like the Chitons, by shelly granules deposited in the 

 horny layer. Some of the groups are ovoviviparous. The great, 

 Brazilian snails lay eggs with hard shells, as large as a pigeon's. 

 In some groups, the shell is little more than horny skin ; and in many, 

 the animal is too large to be withdrawn into it. Some families indeed 

 have no shell at all, or only a plate protecting the most delicate 

 organs. ' The tongue-membrane is not a long ribbon as in the sea- 

 shells, but a short broad horny layer ; partly spread over the soft 

 tongue partly curled up at the side. It is covered with an enormous 

 number of minute square teeth, very similar in pattern, and looking 

 not unlike a tesselated pavement, with raised knobs. 



