4 I2 



ANIMAL LIFE-HISTORIES 



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c.r. 



Fig. 935. Trochosphere Larva 

 of Tusk-Shell (Dentalium}, en- 



m.f., mantle-folds, between which 

 is the foot. 



which resemble the larvae of marine annelids in some respects 



(fig- 935)- 



SNAILS AND SLUGS (GASTROPODA). It will be convenient to 



consider in succession marine, freshwater, and terrestrial members 



of this group, afterwards adding some re- 

 marks on Gastropod Dwellings. 



Egg -laying and Protection of Eggs. 

 Among marine forms there are some very 

 careless parents, such as Common Limpets 

 (Patella) and Sea- Ears (Haliotis), in which 

 the eggs are simply discharged into the 



, . , 1 r r 1 



surrounding water, where but few of them 

 survive the many dangers by which they 

 are encompassed. This casual arrangement 

 is, however, exceptional, and the eggs are 

 usually laid in batches, some provision being also made for 

 their welfare up to the time of hatching. In Sea- Slugs, Wing- 

 Footed Snails (Pteropods), and Heteropods, the "spawn" is a 

 jelly-like mass in which numerous eggs are imbedded. Such 

 masses may be either irregular clumps, or else of more definite 

 form. They may be cord-like, as in the Sea- Hare (Aplysia), or 

 shaped like a spiral band, as in the Sea- Lemon (Doris), or else 

 of some other regular shape. Most of the Sea-Snails lay their 

 eggs in capsules of various kind, which also contain 

 a nutritious fluid for the benefit of the developing 

 embryos, much as in the cocoons of Earth-Worms 

 (see p. 360). Among the common objects of our 

 shores are the little stalked egg -capsules of the 

 Purple-Shell (Purpura lajnUus, fig. 936), of which 

 a large number may be produced by the same 

 individual. Each of them contains from 400 to 600 eggs, of 

 which, however, only from 10 to 16 attain full development, 

 while the rest are used as food by their successful brethren. 

 More striking objects are the egg -capsules of the Common 

 Whelk (Buccinum undatum, fig. 937), of which from 150 to 170 

 are aggregated together into a rounded mass. Each capsule 

 contains from 5 to 30 eggs, of which only a small number hatch 

 out. In certain other species, e.g. the Dog- Whelk (Nassa), all 

 the eggs complete their development. Curious spirally -rolled 

 bands, which are not infrequently cast up on the shore, are the 



ca ps uies 93 of Pu 



