3 2 4 MOLLUSCS. 



retained within the parent shell until hatched. The ova of some of the gastropods, 

 enclosed in capsules, are deposited in masses, and some of these form very remark- 

 able 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 molluscs 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. 



Molluscs are both vegetable and animal feeders, but probably by 

 far the greater number of gastropods are carnivorous. Bivalves 

 imbibe a mixed diet of infusoria and microscopic vegetables. The carnivorous 

 species of gastropods principally attack other kinds of shell-fish, 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 earth-worms. 



Most molluscs which are provided with a more or less distinct 

 Organs of Sense. 



head, namely, the cephalopods and gastropods, are furnished with 



visual organs, but the majority of bivalves (Pelecypoda) are sightless. Although 

 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- 

 molluscs appear to recognise their proper vegetable food by the smell as well 

 as the taste, and the carrion-feeding whelks are probably attracted by odour. 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. 



Molluscs exhibit various ways of progression. Some are free- 

 swimmers, like the cuttle-fishes 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 onwards 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, 

 Sctxicava, 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. 

 Uses in Nature Molluscs form a large item in the food of many mammals, birds, 



and to Man. reptiles, and fishes. Terrestrial forms are devoured by rats, ducks, 



