HABITS AND ECONOMY OF THE MOLLUSCA. 145 



life must be veiy variable. Many of the aquatic species are 

 annuals, fulfilling the cycle of their existence in a single year; 

 whole races are entombed in the wintry tide of mud that grows 

 from 3^ear to year in the beds of rivers, lakes and seas ; thus, in 

 the Wealden clay we find layer above layer of small river-snails, 

 alternating with thin strata of sediment, the index of immeas- 

 urably distant years. Dredgers find that whilst the adults of 

 some shell-fish can be taken at all seasons, others can be obtained 

 late in the autumn or winter only ; those caught in spring and 

 summer being young or half-grown ; and it is a common remark 

 that dead shells (of some species) can be obtained of a larger 

 size than any that we find alive, because thQj obtain their full 

 growth at a season when our researches are suspended. Some 

 species require part of two years for their full development : the 

 young of the Doris and Eolis are born in the summer time, in 

 the warm shallows, near the shore ; on the approach of winter 

 they retire to deeper water, and in the following spring return 

 to the tidal rocks, attain their full growth early in the summer, 

 and after spawning-time disappear. 



The land snails are mostl}^ biennial ; hatched in the summer 

 and autximn,the3'^ are half-grown by the winter time, and acquire 

 their full growth in the following spring or summer. In con- 

 finement, a garden-snail will live for six or eight years ; but in 

 their natural state it is probable that a great many die in their 

 second winter, for clusters of empty shells may be found, 

 adhering to one another, under ivied walls, and in other sheltered 

 situations ; the animals having perished in their hibernation. 

 Some of the spiral sea-shells live%i great mau}^ years, and tell 

 their age in a very plain and interesting manner, by the number 

 of fringes (varices) on their whorls ; the contour of the Ranella 

 and Murex depends on the regular recurrence of these ornaments 

 which occur after the same intervals in well-fed individuals, as 

 in their less fortunate kindred. The Ammonites appear by their 

 varices, or periodic mouths, to have lived and continued growing 

 for many j^ears. 



Many of the bivalves, like the mussel and cockle, attain their 

 full growth in a year, but various facts show that the adult size 

 can be attained within a shorter period. Thus, a ship just 

 careened and freshly bottomed left Marseilles, and in 48 days 

 arrived at the River Gambia, where it remained 68 days, and in 

 86 days additional made the return voyage : which had lasted in 

 all 202 days. Arrived at Marseilles, Mytilus afer, Avicula 

 Atlantica and Ostrea denticidata, fully grown, were found 

 attached to its bottom. These three species iDelong to the 

 African fauna, and thus had acquired their adult size within five 

 months after the earliest period at which, as young shells, they 

 could have attached themselves (Petit). Fischer states that in 



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