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 3-1/2 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. [Illustration: _PLATE XVIII_ _Bathynomus giganteus_, ABOUT ONE-HALF NATURAL SIZE. (_From Lankester's "Treatise on Zoology," after Milne-Edwards and Bouvier_)] 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. The low temperatures prevailing in deep water, even in tropical seas, render it possible for many Crustacea to live there which are closely allied to, or identical with, species occurring in shallow water in the colder seas of the North and South. Many examples of this are mentioned by Dr. Alcock in his discussion of the deep-sea fauna of Indian seas; for example, the Lobster _Nephrops andamanicus_, found at depths of 150 to 400 fathoms in the Indian seas, is very closely allied to the Norway Lobster (_Nephrops norvegicus_) of our own coasts. To some extent this fact affords an explanation of the phenomenon that has been called "bipolarity" in the distribution of marine animals. It has been observed that certain families, genera, and even species, are found in the Arctic and Antarctic seas, although they seem to be entirely absent from the intervening tropical zones. In some cases, however, it has been found that these forms occur in the deep sea in the warmer regions where the cold water offers them a connection between North and South without any great difference of temperature.