3292 THE MOLLUSKS OR SHELLFISH 



are retained within the parent shell until hatched. The ova of some of the gastro- 

 pods, inclosed in capsules, are deposited in masses, and some of these form very re- 

 markable and complicated structures. The number of eggs contained in some of 

 these clusters is enormous. As many as forty thousand have been estimated in a 

 mass, deposited by a single squid. The common whelk occasionally also piles up an 

 enormous heap of capsules, as many as five or six hundred being massed together, 

 each capsule containing several hundred eggs. Land snails, in comparison with 

 marine forms, produce comparatively few eggs. Some of those deposited by the 

 large South- American species are in a few cases half an inch to an inch in length, 

 and have a strong calcareous shell. On the contrary, the productive power of some 

 bivalves is enormous, the ova being counted not by hundreds but by hundreds of 

 thousands, and even millions. The ova of mollusks may be gradually developed 

 into the form of the adult, or there may be a free-swimming ciliated larval stage, 

 or a special larval form as in the fresh- water mussel. 



Mollusks are both vegetable and animal feeders, but probably by 

 far the greater number of gastropods are carnivorous. Bivalves im- 

 bibe a mixed diet of infusoria and microscopic vegetables. The carnivorous species 

 of gastropods principally attack other kinds of shellfish, bivalves being especially 

 appreciated. Some, however, like the common whelk, will feed on dead fish and 

 carrion of any description. Many of them are mere cannibals, and attack their own, 

 kith and kin. Out on the high seas the glassy Carinaria enjoys the succulent jelly- 

 fish, and the squids and cuttles are a terror to many pelagic fishes. The octopus, 

 like the gastropods, is partial to a bivalve meal, and a repast on shrimps and other 

 crustaceans is a daily occurrence. Most land shells are herbivorous, but a few are 

 carnivorous, preying chiefly upon their plant-eating relations, and one curious slug~ 

 lives exclusively on living earthworms. 



Most mollusks which are provided with a more or less distinct head, 

 namely, the cephalopods and gastropods, are furnished with visual or- 

 gans, but the majority of bivalves (Pelecypoda) are sightless. Al- 

 though an auditory apparatus exists, they appear almost insensible to sound. It is 

 certain that most forms are endowed with the sense of smell, although the anatomist 

 has frequently a difficulty in discovering the position of the olfactory organ. Land 

 mollusks appear to recognize their proper vegetable food by the smell as well as 

 the taste, and the carrion-feeding whelks are probably attracted by odor. The 

 senses of smell and taste are probably but imperfectly developed in the bivalves, 

 which scarcely possess the power of selection as regards their food. 



Mollusks exhibit various ways of progression. Some are free 

 swimmers, like the cuttlefishes and squids, pteropods, heteropods, and 

 a few bivalves; others are mere crawlers, like snails and whelks; and some creep 

 along, but beneath the surface of the water. The Melampus moves onward 

 after the fashion of a looper caterpillar, and the bivalves either crawl upon 

 their foot, or progress by a jerking or leaping movement. Many species, like 

 the limpet, Saxicava, and Pholas, are very sedentary in their habits, and 

 others, which in their early career are active, in after life are stationary in their 

 permanent abodes. 



