eighty pairs) along the sides of the sea cradle's foot, 

 or to the ladder-like cross connections between the 

 two separate but parallel nerve cords from which the 

 class Amphineura takes its name (Greek amphi, on 

 each side, and neiira, a nerve). None of the other in- 

 ternal organs shows a pattern suggesting repetition. 



Although they lack eyes, most chitons are sensitive 

 to light and feed only in hours of darkness. Many of 

 them return to the same site whenever not actually 

 foraging. Others apparently never leave the "home 

 spot." The commonest sea cradle of exposed coral- 

 line- and mussel-covered rocks along the Pacific 

 coast of America {Nuttallina califoniica) remains 

 fixed in this voluntary way. Repeated pounding by 

 the waves and erosion aided by this 1 '/2-inch mollusk 

 produce depressions the size and shape of its spiny 

 girdle and create little eddies with the ebb and flow 

 of each wave. The eddies deposit seaweed debris in 

 the depressions, bringing food to the animals in this 

 way. Nuttalliim is believed to survive for more than 

 twenty-five years in this sedentary life, and the de- 

 pressions are used by generation after generation of 

 this mollusk, probably for thousands of years. 



The giant of all sea cradles is Cryptochhon stelleri, 

 whose brick-red girdle completely covers the shell 

 valves. It is called the sea boot or gumboot, and in- 

 habits rocks from Bering Strait to California and to 

 Japan. A 13-inch specimen may be 6 inches wide. 



On the intertidal coasts of Alaska, the most abun- 

 dant sea cradle is the large, dead-black Katharina 

 tiinicata (Plate 39), whose valves barely show 

 where the expanded girdle leaves little heart-shaped 

 gaps along the back. It is common on both sides of 

 the North Pacific, seeming to prefer rocks that form 

 ledges about halfway between mid-tide and low. It 

 tolerates full sunlight longer than most chitons. 



Along Atlantic coasts the sea cradles are smaller 

 in high latitudes, and the % -inch species of Lepido- 

 chiton tend to be the chief ones found with a clean- 

 appearing zone of girdle platelets. Chaetopleitra 

 apiciilata, of about the same size, is common below 

 low-tide mark from Cape Cod to Florida; it has a 

 hairy girdle and a keel down the middle of the shell 

 valves. In tropical waters, far larger chitons tolerate 

 intense sunlight for hours while exposed by the tide. 



Each sea cradle is either a male or a female. Many 

 of them congregate in springtime, which is spawning 

 time, and the females may each lay two long, spiral 

 strings of eggs in jelly. The egg strings of Isclmo- 

 chiton magdalensis on the California coast average 

 31 inches in length, and have been found to contain 

 between them from 100,000 to 200,000 eggs. The 

 young emerge as swimming larvae, but within a cou- 

 ple of hours they settle and transform to the shell- 

 bearing adult. 



In addition to the sea cradles or chitons with 

 plates (the "loricates" of order Polyplacophora), the 



class Amphineura includes some seemingly degener- 

 ate, shell-less animals (order Aplacophora). These 

 are the 1-inch, wormlike solenogasters, which live in 

 the sea at depths greater than ninety feet, creeping 

 over hydroids and corals upon which they feed. 



Each solenogaster has a cylindrical body with a 

 mouth at one end and an anus between two project- 

 ing gills at the other. If a foot is present, it consists 

 only of a narrow ventral groove. Apparently all 

 solenogasters begin as a larva with seven transverse 

 limy plates on the back and a radula in the mouth. 

 But the plates, and in some cases the radula as well, 

 are lost at maturity. The body is then clothed in limy 

 spicules that project from the enveloping mantle. 



The Snails and Sluos 



o 



{Class Gastropoda) 



When a person describes something as being a flat 

 spiral, he usually compares it with a watch spring, a 

 butterfly's tongue, or a snail shell. All snail shells to- 

 day do have a spiral origin, even when (as among 

 the limpets) no outward trace of this may remain. 

 Back at the beginning of the fossil record, however, 

 the earliest known snails had straight shells or long, 

 curved, conical ones suggesting today's tusk shells, 

 except that they were closed at the small end. 

 Through adoption of a spiral shape, a snail can carry 

 within the armor of the shell a long, pointed mound 

 of body and manage it in a neatly poi table form. 



Most snails glide about on the large, flat, foot por- 

 tion of the body and show a definite head end, often 

 with eyes and sensitive projections (tentacles). Usu- 

 ally, when danger threatens, the snail can withdraw 

 into the safety of the shell, pulling in first the tenta- 

 cles and head, then the complete foot. A good many 

 snails even carry on the side of the foot a flat plate 

 which forms a hard door (operculum), closing the 

 shell completely after the animal is inside. 



That snails and slugs appear to creep on their 

 belly surfaces is recognized in the class name (from 

 gaster, the belly, and pes, podos, a foot). The gastro- 

 pod combines a skidding action of the rim of the foot 

 along a sheet of mucus secreted at the anterior end, 

 with movement of the sole proper in a series of 

 waves. Transverse bands of the sole alternately sup- 

 port the weight of the animal and are moved back- 

 ward in a stretching action, and then are lifted clear 

 to shift forward again, ready to take part in the next 

 downward cycle. 



The '/3-inch chink shells (Lacuna), which super- 

 ficially resemble periwinkles, creep about on sea- 

 weeds and eelgrass with a ditTerent gait. The foot is 

 grooved lengthwise, and the snail waddles — advanc- 

 ing one side of the foot and then the other, swaying 



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