Another order of the crustaceans—the Copepoda, or oar-footed group—is so called on account of the bristled feet that are employed after the manner of oars when the creatures are swimming.
These Copepods are small animals, so small indeed that the compound microscope is generally necessary merely for the examination of their external characters. Many species inhabit fresh water, and the study of the group is more commonly pursued by the investigator of fresh-water pond life than by the sea-side naturalist. However, marine species are abundant, and may be captured in the open water or in rock pools by means of a muslin net. As with the last order, some degenerate from the comparatively complicated free-swimming and eyed larval state to blind and limbless parasites that feed on the bodies of fishes and are known as fish lice.
The body of the typical copepod is distinctly segmented, and the head and thorax are both enclosed in a hardened buckler. It has two pairs of antennæ, two pairs of foot jaws by which it captures its prey, and four or five pairs of bristled feet for swimming. The jointed abdomen has also a tuft of bristles at its extremity. The annexed illustration represents some marine species, and will serve to show the general features of the order.
Fig. 196.—A group of Marine Copepods, magnified
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.
Fig. 197.—A group of Ostracode Shells