1.592 



ZOOLOGICAL SOCIETY BULLETIN 



t 



Left: OYSTER DRILL; Center: SEAWEED SNAIL 

 Right: PERRIWINKI.E 



tion by the extrusion of mucus, "affording a 

 semi-solid rod against which to push." This is 

 probably the method of locomotion of many 

 snails. However, considerable difference of 

 opinion exists on this point, some observers 

 maintaining that snails move by muscular con- 

 traction and relaxation of the foot, other theo- 

 ries variously holding that they effect locomo- 

 tion by means of a blood How to the foot tissues ; 

 by peristaltic contraction of the foot in undula- 



fe m M 



POTOMAC SNAILS 



foot 



torv waves; by action of the cilia on the 

 and not by motion of the foot itself, etc. 



The mud snail requires considerable oxygen. 

 Its siphon supplies the gills with oxygen from 

 the water when submerged, while, when left dry 

 at low tide, air is inhaled through the siphon 

 and life sustained as long as there is some water 

 remaining in the mantle cavity. Moisture is 



ROCK SNAILS 



necessary to its existence, and it dies much 

 sooner when deprived of it than when prevented 

 from rising to the surface for air. Indeed, it is 

 a burrower. and will bury itself in the mud and 

 remain quiescent for a considerable period. 



The sexes are separate but there are no ex- 

 ternal distinguishing marks. Lnder natural 

 conditions eggs are laid during the warm 

 months. In small aquaria they appear at all 

 seasons on the sides of the tank or attached to 

 the sea-lettuce. The egg capsules, shaped 

 like miniature urns, are dirty white, semi-trans- 

 lucent, and may contain as many as seventy-five 

 ova. The young hatch as invisible, free-swim- 

 ming veligers, and. for several days prior to 

 leaving the egg, the cilia of their large "ears," 

 under magnification, can be observed already in 

 rapid vibration. The shell is only partly formed 

 at hatching, but a couple of weeks' growth com- 

 pletes it. and the tiny creature now having lost 

 its means of locomotion, no longer swims, but, 

 snail-like, crawls on its large foot. 



Typical of snails, this species is a scavenger. 

 It takes food only under water. It will eat al- 

 gae and sea-lettuce but does little damage to 

 the plant life of the home aquarium. Its favor- 

 ite food is fresh meat in the shape of other mol- 

 luscs, worms, shrimps, crabs and dead fish, and 

 it will assiduously clean up all the scraps from 

 the aquarium, only the eel surpassing it in scav- 

 enging ability. For a slow-moving animal, bur- 

 dened with a shell uncommonly heavy in a mol- 

 lusc of its size, it is extremely voracious. It is 

 recorded that several together have been ob- 

 served to capture a clam-worm (Nereis) and 

 devour it alive, though we suspect that the worm 

 must have been caught napping. 



The mud snail is nearly black. The upper 

 surface of its foot is heavily pigmented, and the 

 under or crawling surface is pale gray. The 

 shell is marked with deep, brownish grooves, 

 and is so much subject to erosion that some 

 specimens are worn away to the innermost layer. 

 When full grown it measures about an inch in 

 length. The operculum is small, only partly 

 closing the opening of the shell when the body 

 is withdrawn. 



The animal often carries around a pretty lit- 

 tle roof garden of waving green algae, and 

 occasional specimens are so densely overgrown 

 with the fringe-like plant Enteromorpha, that 

 at first glance they might be mistaken for sea 

 mice. 



The species is unique in the possession of a 

 long inquisitive siphon that can be turned in 

 anv direction, and which, traveling on before 



