Place them in the indoor aquarium, or in any shallow vessel containing just sufficient sea-water to cover them, and carry out your observations with the aid of a hand lens. They will soon open the inner cone of their many-valved shell, and slowly protrude six pairs of gracefully curved and delicately-feathered appendages which, as previously stated, are attached to the thoracic portion of the body. Then, with a much more rapid movement, the appendages will be withdrawn, and the shell closed. These alternate movements are continued incessantly, and are the means by which the animals provide themselves with both food and air. The reader should also obtain some specimens of the larger species for the examination of the shell, the structure of which is interesting and, of course, peculiar to this order. [Illustration: FIG. 195.--THE ACORN BARNACLE (_Balanus porcatus_) WITH APPENDAGES PROTRUDED] In general structure and habits Barnacles are very similar to the acorn barnacles, except that the body is supported on a tough stalk, which, as we have already stated, is the modified anterior portion of the animal. These animals also may be easily kept alive and examined in the indoor aquarium. They are not creatures of the sea shore, but may often be obtained on masses of timber that have been washed ashore, or from the bottoms of ships that have been placed in the dry dock for repairs. 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. [Illustration: FIG. 196.--A GROUP OF MARINE COPEPODS, MAGNIFIED]