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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 
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, 
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. 
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, 
Organs of Sense. 
Locomotion. 
