PHYLUM MOLLUSCA 



301 



shaped and adapted for ploughing; 

 usually two gills on either side of 

 the mantle cavity. Entirely aquatic. 

 Order 1. Taxodonta. The mantle mar- 

 gins are united ventrally and 

 posteriorly with openings for 

 siphon and foot; gills usually 

 absent, and hinges reduced or 

 absent. Shells with many simi- 

 lar teeth on hinge margin; usu- 

 ally two equal adductor muscles. 

 Entirely marine. Ex. Yoldia 

 limatula in deep water from 

 Connecticut northward. 

 Order 2. Anisomyaria. The mantle mar- 

 gins are usually separate, ven- 

 trally and posteriorly; and the 

 siphons are lacking, or only 

 slightly developed. Teeth var- 

 ious; anterior adductor muscle 

 small or none; posterior adduc- 

 tor, a large powerful muscle. 

 Ostrea, edible oysters; Mytilus, 

 sea mussel; Pecten, frcquentlv 

 called scallop; Pinctada (for- 

 merly Margaritana) are marine 

 forms, and sometimes called 

 pearl oysters. 

 Order 3. Eulamellibranchia. The mantle 

 more or less connected ventrally 

 and behind; siphons generally 

 well developed. Hinge teeth 

 usually small in number and 

 unlike in form; adductor mus- 

 cles equal, or anterior muscle 

 smaller. Marine, brackish, and 

 fresh water. Exs. Mercenaria 

 (formerly Venus), quahog or 

 hard-shell clam; Ensis, razor 

 clams; Mya aenaria, sand clam; 

 Teredo, shipworm; Pisidium, 

 free-living in fresh water; and 

 Unio, Anodonta, Lampsilis, 

 fresh-water clams, shells used 

 for buttons. 

 Class 5. Gastropoda (Gr. gastro, belly; pons, 

 foot). Snails, slugs, whelks, etc. 

 (Figs. 170, 181, and 182). Foot flat 

 for creeping, head distinct, with eyes 

 and tentacles; shell if present of one 

 piece (one valve); shell usually spiral 

 but uncoiled, reduced, or absent in 



some; radula present in all but a few 

 parasitic species; trochophore and 

 usually vcliger larvae. 

 Subclass 1. Prosobranchia. Mostly marine 

 snails, but fresh-water and land 

 forms are represented (Fig. 

 170). Respiration usually by 

 gills which are situated in the 

 mantle cavity anterior to the 

 heart. If gills arc absent then 

 respiration may be by means of 

 mantle, or pulmonary chamber. 

 Exs. Strombus gigas, giant 

 conch; Helicina orbiculata, a 

 terrestrial southern species 

 which is frequently arboreal; 

 Fissurella, keyhole limpets, aba- 

 lones, oyster drills; and^Crepid- 

 ula, slipper or boat shells. 

 Subclass 2. Opisthobranchia. Strictly ma- 

 rine (Figs. 170 and 182). Small 

 shell or none; gills, if present, 

 are gituated posterior to the 

 heart. Hermaphroditic. Exs. 

 Clione, pteropods or sea butter- 

 flies, the foot may be expanded 

 into two fins used in swimming; 

 the sea hare and nudibranchs 

 (snails without shells). 

 Subclass 3. Pulmonata (Fig. 170) (Gr. 

 pulmones, lungs). Mostly fresh- 

 water and land snails. No gills; 

 mantle cavity serves as a pul- 

 monary sac (lung); shell usu- 

 ally present, sometimes rudi- 

 mentary or absent; one or two 

 pairs of tentacles; mostly ter- 

 restrial, many in fresh water, a 

 few marine; mostly vegetarian, 

 a few carnivorous. Exs. Lym- 

 naea stagnalis, fresh-water spe- 

 cies; Helix, European garden 

 snails, introduced into America; 

 Anon, slugs with no shell; 

 Limax, with rudimentary shell 

 in mantle; and Testacella hali- 

 otidea, a slug that lives in 

 greenhouses and preys on earth- 

 worms. 

 Class 6. Cephalopoda (Gr. kephale, head; 

 pons, foot). Squids, cuttlefish, oc- 

 topuses, and nautili (Fig. 170). All 



