Page:Popular Science Monthly Volume 22.djvu/381

Rh in any general theory of life demands, a short account of one of the less complex instances will not be out of place.

Entoconcha is an extremely simple, worm-like animal, which lives, as a parasite, inside the body of an holothurian. It is fastened, by a button-like head, into a perforation in the wall of the digestive tract of the holothurian in such a way that, while its mouth opens into the digestive cavity, its long, contorted body hangs in the body cavity of its host, so that it is bathed by its fluids, and protected by its bodywall. As the digested food passes by its mouth the animal sucks it into its rudimentary stomach, and, as all its wants are thus provided for, the conditions of its life are extremely simple, and its bodily structure exhibits a corresponding simplicity, and it may be described as a long, cylindrical, worm-like animal, with a simple, pouch-like stomach which opens by the mouth at the anterior end, and occupies about one half the length of the body, while the other half is filled by the equally simple organs of reproduction. It is as lowly organized as the simplest of parasitic worms, and it is only by a study of its development that we learn it is not a worm at all, but a gasteropod mollusk, which has become degraded or simplified to adapt it to a parasitic life. The ordinary gasteropods, the snails, conchs, etc., are animals of quite high organization. They are usually provided with a protecting shell, and their organs of locomotion are well developed. In connection with a specialized muscular system, they possess a well-marked and complex nervous system, as well as sense-organs, such as eyes, tentacles, and hearing organs. The digestive organs are quite highly specialized, and consist of parts which bear a close physiological resemblance to those of a vertebrate. The food is masticated by a very complicated system of jaws and teeth, and after it has been mixed with a secretion, which is poured into the mouth by salivary glands, it passes through a long muscular œsophagus into the stomach, where it is acted upon by fluids furnished by a liver and other glands, before it passes into the long, convoluted intestine, where the nutritive portions are absorbed into the blood, while the waste products are discharged from the body through the anus. The nutritive matter is driven to all parts of the body, with the blood, through a system of arteries and veins, by a pulsating heart, and during a part of its course the blood passes through respiratory organs, where it is aerated by contact with the air or water. The waste products are excreted from the blood by renal organs, and the organs of reproduction are extremely complicated and highly specialized.

The animal which has been seen to hatch from the egg of entoconcha is a young gasteropod, which, like the young of ordinary forms, has a spiral shell, a muscular locomotor foot, tentacles, eyes, hearing organs, and a nervous system like that of other gasteropods at the same stage of growth. The digestive tract is divided into regions, like those of an ordinary gasteropod, and the young entoconcha