With regard to some of the other larval forms, it is possible to speak with a little more confidence. There are good grounds for believing, apart from the evidence of development, that the Lobster and its allies have descended from Crustacea which, like the existing Euphausiacea, possessed swimming branches (exopodites) on the thoracic legs; and there seems no reason to doubt that the "schizopod" larva of the Lobster does recapitulate this stage in the evolution of the race. On the other hand, it is impossible to believe that any of the ancestors of the Shore Crab resembled, even remotely, the zoëa stage with which the life-history of the individual now begins. CHAPTER V CRUSTACEA OF THE SEASHORE The tract of seashore which is laid bare by the retreat of the tide offers on most coasts a rich collecting-ground to the student of Crustacea. In places where shelving, weed-covered rocks run out to sea, innumerable Crustacea have their home in the rock-pools, or lurk in crannies awaiting the return of the tide. On sandy beaches, at first sight apparently barren of life, a closer search will reveal a whole fauna, amongst which burrowing Crustacea of various orders are prominent. Further, the shore collector will find from time to time stray specimens of forms that have their proper habitat beyond low-tide mark, and occasionally their remains are thrown in quantities on the beach by storms. It is convenient, therefore, to treat the Crustacea of the shore as a sample of those inhabiting the shallower waters of the ocean. In these shallower waters--down to the limit where light no longer penetrates from above, where vegetable life ceases, and where the strangely modified inhabitants of the deep sea begin to appear--the sea-bottom is perhaps the most densely populated of all parts of the earth's surface. Nowhere, at all events, do we find so wide a range of animal forms, from the simplest organisms (Protozoa) up to highly-organized Vertebrates. Nowhere, perhaps, is the struggle for existence more keen, and it is not without justice that some naturalists have regarded the shallow waters of the sea as "one of the great battle-fields of life," where, in the long course of evolution, the main branches of the animal kingdom have had their origin. Conspicuous among the animals of this region are Crustacea of all sorts and sizes. To identify all the species that may be obtained in a single haul of the dredge in British seas would sometimes be a hard task even for the most expert student of the group. Our present purpose, however, is not to compile a faunistic catalogue, but merely to give some idea of the endless diversity of form, and to note a few of the "shifts for a living"--of the ways in which structure and habit are adapted to the conditions of life in the Crustacea of the shore and of shallow water.