SPECULATIVE ZOOLOGY. 367 



cance 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 body- 

 wall. 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 devel- 

 opment 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 connec- 

 tion 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 re- 

 semblance 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 oesophagus 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 ento- 

 concha 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 



