494 MOLLUSCA 



neurons or orthoneurous. The sensory organs are a pair of eyes, one or 

 two pairs of tentacles, a pair of lithocysts in the foot, one or two osphradia 

 close to the ctenidia, and the rhinophores of the opisthobranehs, the last 

 two organs being olfactory in function. The osphradia are absent in most 

 snails, which lack ctenidia, but not in all. The rhinophores are the pos- 

 terior pair of tentacles of the opisthobranehs. 



The single gonad empties its products in certain prosobranchs 

 (Patella) through the kidney, but in most gastropods through special 

 genital ducts, to the outside, the genital pore being in the mantle cavity 

 in prosobranchs and outside of it in opisthobranehs and pulmonates. 

 Most prosobranchs are unisexual, while the pulmonates and opistho- 

 branehs are hermaphroditic. The majority of snails are oviparous, 

 and many of them lay their eggs in protective capsules or a jelly; 

 a few are viviparous. Most marine snails leave the egg in the form 

 of the veliger larva; land snails lay eggs, which are covered with a 

 shell, in holes in the ground or similar places, and are born with the 

 form of the parent. 



Distribution and Habits.— Most gastropods are aquatic animals, and 

 those which are terrestrial usually require a moist environment. A few 

 are parasitic, either on the outer surface (Stilifer) of star-fishes and other 

 animals or as internal parasites (EntoconcJia) in holothurians. Many are 

 of use to man. Helix pomatia, the imported, so-called French snail of the 

 New York markets, and many others, being used for food. The shells 

 also serve often for ornaments and are manufactured into buttons 

 and other articles. In Africa and other countries immense quantities of 

 cowries {Cyprcea moneta), and several other shells, are used as money. 



Hist or I/. — The class Gastropoda was created by Cuvier in 1795, who 

 included under it, among others, the orders Nudibranchiata, Tectibranch- 

 iata, and Pulmonata. The Pteropoda he created later, but as a separate 

 class equivalent to the Gastropoda. Lamarck in 1812 created the Hetero- 

 poda, and gave the group equal rank with the Gastropoda. Milne- 

 Edwards in 1846-1848 created the orders Opisthobranchiata and Proso- 

 hranchiata. The Pteropoda were believed by Leuckart to be the ancestors 

 of the Cephalopoda, the arms of which he supposed to have come from 

 pteropod fins, and Lankester has classified them under the Cephalopoda. 

 They were, however, universally placed in a class by themselves until 

 Pelseneer, in 1888, showed their affinities to the Opisthobranchiata. The 

 same author has also utilized the condition of the pleuroviseeral connectives 

 in classification, dividing the Gastropoda into two groups, the Euthyncura, 

 with the Opisthobranchiata and Pulmonata, and the Streptoneura, with 

 the Prosobranchiata, a proceeding which has been adopted in some text- 

 books. The class contains 49,000 species, grouped in 3 orders. 



