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In many deep-sea Crustacea the eggs are of very large size, indicating that the young are hatched in an advanced stage of development. For example, in the numerous species of the genus Munidopsis the eggs are always large and correspondingly few in number, in striking contrast to the closely allied genus Galathea, from shallow water, in which the eggs are small and very numerous. Alcock mentions that a deep-sea Prawn of the genus Psathyrocaris, although only about 3½ inches long, has eggs nearly a quarter of an inch in length. It would seem that, in some way or other, the conditions are unfavourable for a free-swimming larval life; but they cannot be altogether prohibitive, for there are a good many characteristically deep-sea Crustacea, such as the Eryonidea, that have small eggs and presumably a larval metamorphosis.

PLATE XVIII

Bathynomus giganteus, ABOUT ONE-HALF NATURAL SIZE.

(From Lankester's "Treatise on Zoology," after Milne-Edwards and Bouvier)

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The uniformity of the physical conditions over vast areas in the deep sea is no doubt the cause of the enormously wide geographical range of many species of deep-sea animals. There are many examples of this among Crustacea, and they are added to by every deep-sea dredging expedition. For example, the giant Isopod Bathynomus ([Plate XVIII].) was first discovered in West Indian seas, and the same species has since been dredged near Ceylon, while a second species has been found off the Japanese coast. Of the strange lobster-like Thaumastocheles ([Fig. 44]), mentioned above, only four specimens are known—one dredged by the Challenger in the West Indies, and three others more recently brought from Japan.