the blood, and is so thin and delicate, and so richly supplied with blood-vessels, that, when moistened, it readily admits of the interchange of gases. During the breeding-season the abdomen of the female frog is so distended by eggs that there is no room for the inflation of the lungs, and respiration is entirely carried on through the skin.
In those marine and aquatic animals of small size whose bodies are not covered by impervious shells, respiration may take place, as in plants, over the whole surface of the body; and in many groups, the larger and protected members of which are furnished with highly specialized respiratory organs, these organs may be entirely wanting in the smaller naked forms. This is the case, for instance, with many of the naked mollusks.
In place of a closed system of vessels for the circulation of the blood, this fluid may, in parts of its course, find its way through the spaces among the viscera, and in the Tunicata a pulsating heart keeps in motion a hæmal fluid, which is nowhere confined by distinct blood vessels, but fills and circulates through all the intra-visceral spaces of the body. In most mollusks, again, themovements of the various parts of the soft body aid the heart in maintaining the circulation; and in the Polyzoa both vessels and heart are lacking, and the movements of the blood are due to muscular contractions, aided in many cases by cilia.
The salivary glands are frequently wanting in marine animals; the water swallowed with the food taking the place of saliva. The liver and other appendages to the digestive tract are wanting in many of the lower animals, and a parasitic life may entirely do away with the need for them, even in an animal which belongs to a highly-specialized group. The ordinary gasteropod mollusks have digestive organs which are almost as highly specialized as those of a vertebrate; but "entoconcha," a strange parasitic gasteropod, has no jaws or teeth, no salivary glands or liver, no anus, or any other specialized appendage to the digestive cavity, which is a simple pouch, which is not divided into œsophagus, stomach, and intestine, and the animal lives inside the body cavity of a holothurian, attached to the wall of its stomach, and is' nourished by the fluids which it sucks from the digestive cavity of its host. In the sponges and hydroids there is no digestive cavity distinct from the body cavity, and the food is received directly into the latter. Even the mouth is wanting in many parasitic worms, and the liquid food is absorbed through the outer surface of the body, precisely as in plants, and some parasites put forth root-like processes which penetrate the tissues of the host and absorb its juices.
The nervous system and organs of special sense are wanting in many of the lower animals, and in the fresh-water hydra the reproductive organs are not formed in specialized internal glands at definite points, but resemble those of plants in making their appearance at many points upon the surface of the body.