The ova of many molluscs are deposited in masses enclosed in capsules. Some of them are very wonderful and complicated structures. Those of the Cuttles and their allies are clustered like grapes, each capsule containing but a single embryo; but in the Calamaries or Squids they form a radiating mass of elongated sacks, each containing from thirty to two hundred eggs, and it has been estimated that one of the spawn-clusters of the Common Squid (_Loligo vulgaris_) contains as many as 40,000 ova. Everybody knows the spawn-cases of the Common Whelk, found so abundantly on the sea-beach, consisting of a large number of yellowish capsules, heaped one upon another and forming an irregularly rounded mass. As many as five or six hundred capsules may be piled together in a single heap, each capsule containing several hundred eggs, of which perhaps only thirty or forty are hatched. In other genera, as _Tethys_, _Doris_, _Eolis_, &c., the eggs are contained in a spirally rolled ribbon or strap-like structure; and some of the _Naticæ_ build a somewhat similar capsule, composed of the eggs cemented together by sand and a gelatinous material, the whole forming two-thirds of a circle narrowed at the upper part. Terrestrial Molluscs deposit, in comparison with their marine relations, but very few eggs. They are sometimes covered by a thin soft skin, but in certain groups, such as the large South-American _Strophochili_ and the African _Achatinæ_, which include the largest of known land-molluscs, they are protected by a hardened calcareous shell, in some instances fully an inch in diameter. The freshwater forms (_Limnæa_ and _Physa_) deposit from thirty to a hundred eggs enveloped in a gelatinous mass. The number of eggs produced by some Bivalves is enormous. The Common Oyster is said to produce a million or more, and the American variety ten, or even sixty, times as many. Some of the River-Mussels are also very prolific, as many as two millions being sometimes the product of a single individual. A small series of the eggs of Land-Snails and of the egg-capsules of some marine Gastropods is exhibited in side table-case C at the side of the Gallery. The ova of Mollusca may be gradually developed into the form of the parent, or there may be a free-swimming larva, which has a circlet of cilia near the anterior pole of its body (so-called “Veliger” larvæ), or there may be special larvæ, as in the case of the Freshwater Mussel, the “Glochidium,” as it is called, which has a toothed bivalve shell by which it can fix itself to fishes. [Sidenote: Duration of life.]