n 



separates it from its rock surface. The merest touch, 

 however, before the shove arrives is enough to in- 

 duce a limpet to exert its full abilities as an animated 

 suction cup. This suction has been measured as 

 seventy pounds to the square inch of foot surface. 

 That it is actually suction can be demonstrated by 

 sliding a thin knife blade between the foot and the 

 stone, letting air or water break the mucus seal; the 

 animal will then be found to have lost all hold upon 

 its support. 



After growth is well under way, the shell of a 

 limpet shows little of its spiral origin. The shell is 

 tentlike as soon as a whorl or two have been pro- 

 duced, and thereafter additions are made evenly all 

 the way around, keeping the shelter bilaterally sym- 

 metrical. A keyhole limpet (Plates 47, 48) has a 

 hole at the peak like the crater of a volcano, and for 

 this reason is sometimes called a volcano shell. The 

 hole originates as a notch in the outer lip of the be- 

 ginning shell. Later growth closes the notch, and 

 the limpet goes on to make the shell symmetrical. 

 But it continues to use the opening for discharge of 

 wastes from the anus and for a current of water 

 drawn under the shell at the anterior end by cilia cov- 

 ering the mantle surface. 



The dunce-cap limpet Acinaea initra of North Pa- 

 cific coasts is the tallest of the true limpets. It reaches 

 a shell length close to 2 inches and a height of 1 inch, 

 and hence is probably the bulkiest of all limpets. 

 Some other lower-spired species cover a greater area 

 of rock. The largest of keyhole limpets is Liicapina 

 crenulata of southern California and Mexico. It 

 reaches a length of 7 inches and a width approach- 

 ing 3. On the Atlantic coast of America, the eastern 

 keyhole limpet FissweUa alteniata ranges all the way 

 from New Jersey into the West Indies and the Gulf 

 of Mexico, but seldom attains a length of more than 

 an inch. 



One of the sea's strangest snails is the wearer of 

 the "purple shell," Janthina fnigilis. This eyeless, 

 violet-colored animal drifts near the surface of all 

 warm oceans, suspended from a raft of air cells in a 

 mat of mucus. Often these creatures advance in large 

 schools, yet the individual members appear able to 

 find enough of their favorite jellyfishes to eat. Each 

 Janthina is an expert at spearing small medusae on 

 a long, prehensile proboscis. 



Probably sea birds are Janthina's chief enemies. 

 Against them it is camouflaged by its own color, and 

 in addition it is armed with a glandular bag full of 

 purple liquid. It expels this fluid into the surrounding 

 water in a cloud, against which it shows no contrast. 

 Janthina produces a floating egg raft too, and usually 

 stays with it until the young snails hatch. Often both 

 Janthina and its rafts are cast upon subtropical and 

 tropical beaches as prizes for the curious. 



Another oddity encountered by the beachcomber 



Limpets that live high on the shore, like Acmaea dig- 

 italis, common along most of the American Pacific 

 coast, must withstand extremes of temperature and 

 drying. ( Oregon. Ralph Buchsbaum ) 



is the sea collar, a capelike thin swirl of sand grains 

 among which the eggs of the moon snail Polinices 

 (Plate 55 ) appear as transparent dots when the col- 

 lar is held up to bright sunlight. The moon snail it- 

 self has a tremendous foot, so large that the heavy, 

 globular shell can scarcely accommodate it. The ani- 

 mal uses the foot as a bulldozer blade while plowing 

 through the surface sediments of sandy beaches, feel- 

 ing for clams it can hold while drilling through the 

 shell to reach their flesh. 



When Polinices is ready to lay eggs, she exudes a 

 film of mucus from the foot and spreads this over the 

 exposed part of her body, adding eggs at the same 

 time as the mucus covering becomes impregnated 

 with sand grains. When the mucus hardens to a 

 leathery jelly, the parent snail slips out of a gap in the 

 sea collar, leaving it on the sandy bottom. If the col- 

 lar washes ashore, it may remain intact in ver)' humid 

 air. But if it dries out, the mucus becomes brittle and 

 the collar extremely fragile. Sand collars are some- 

 times 8 inches across and 2'/2 to 3 inches high. 



Polinices is so obviously too large for its shell that 



[179 



