The sea-side naturalist, intent on the collection of small life, may possibly meet with representatives of two other orders of crustaceans--the _Ostracoda_ or shelled crustaceans, the bodies of which are enclosed in a bivalve, hinged shell; and the _Branchiopoda_, so called because the branchiæ or gills are attached to the feet. [Illustration: FIG. 197.--A GROUP OF OSTRACODE SHELLS] The Ostracodes have two or three pairs of feet which subserve locomotion, but are not adapted for swimming; and two pairs of antennæ, one of which assists in locomotion. The mouth is provided with organs of mastication, the branchiæ are attached to the hind jaws, and the animals have but one eye. Some of these crustaceans inhabit deep water only, while others live in sand between the tide-marks; but several species, belonging chiefly to the genus _Cythere_, abound in rock pools, where they may be readily obtained by scraping the confervæ and corallines with a small muslin net. The branchiopods are free swimmers, and are protected by a buckler-like envelope. Most of them are inhabitants of fresh water, and are popularly known as water fleas. We have figured one marine species, belonging to the genus _Evadne_, which has a colourless body, and a single conspicuous black eye, and is interesting as being the food of the herring. [Illustration: FIG. 198.--_Evadne_] The four orders of crustaceans that have been briefly described belong to the division _Entomostraca_, which signifies ‘shelled insects.’ This term is not a happy one when judged from the standpoint of our present knowledge of animal life, but it must be remembered that, at the time it was applied (1785), spiders and crustaceans were all included in the same class as the insects; and this is hardly surprising when we observe the close relationship of these animals, as shown in their segmented bodies and jointed appendages; for, as we have already shown, the lowly organised parasitic crustaceans which, in the adult state, lose most of their appendages and cease to be distinctly segmented, are more or less insect-like in their larval and free-swimming stage. All the other crustaceans are included under the term _Malacostraca_, or soft shelled, since, although many of them are protected by an exo-skeleton that is hardened by the deposit of carbonate of lime, yet, generally speaking, their coverings are softer than those of the molluscs; and therefore the term _Malacostraca_ was originally applied by Aristotle in order to distinguish them from the animals that are covered by harder and thicker shells. This division of the crustaceans contains wood lice, sandhoppers, lobsters, shrimps, crabs, &c., and consists of two main groups--the Sessile-eyed (_Edriophthalmata_) and the Stalk-eyed (_Podophthalmata_) crustaceans. We shall now consider the Sessile-eyed group, dealing first with the order _Isopoda_ or equal legged, and then the _Amphipoda_, which have appendages adapted both for walking and swimming.