APODA. 643 



and no thread-like appendages at the posterior extremity, of which T. Neptuni is an example ; and Echiurus, which 

 have bristly hairs at the posterior extremity. They inhabit the sands, and are much soug^ht after by fishermen 

 as bait. Sternaspis, has bristles as in the last, and a disc of a horny texture, and surrounded with hairs on the 

 anterior part of the body. The habits of all these are very much the same. 



THE SECOND CLASS OF THE RADIATA. 



THE ENTOZOA, or Intestixal Worms. 



This class is remarkable for by far the greater number being inhabitants of the 

 internal parts of other animals, in which alone they can continue their species, — so 

 that it must be regarded as their natural habitat ; and they must have a use in the 

 economy of nature with which we are quite unacquainted. There is scarcely one 

 animal, especially of the vertebrated classes, which is not infested by several kinds ; and 

 those which inhabit one animal, are rarely found in one of another genus. They are 

 met with most abundantly in the alimentarj' canal, and the ducts which empty their 

 contents into it ; but they occur also in the cellular tissue, and in the parenchyma of 

 the most closely invested viscera, such as the liver and the brain. They are most fre- 

 quent in diseased states of the viscera, and they themselves occasion disease, or, at all 

 events, annoyance ; but they occur even in healthy states. The difficulty of con- 

 ceiving how they could get into places so obscure, and apparently so well protected, 

 and the fact of their never having been found alive except in the interior of living 

 animals, caused it for a long time to be believed that they were products of spon- 

 taneous generation. It has been found, however, by actual observation, that most of 

 them either produce ova or living young ones, and that many of them have the sexes 

 in different individuals. Though some of them attain a very large size, we must sup- 

 pose that the germs are exceedingly minute, and capable of being transmitted through 

 capillary vessels, and apertures too small for being discerned by the naked eye ; and, 

 from the early age at which they are found in some animals, there is reason to con- 

 clude that the germs have been in these anterior to their birth, [though how trans- 

 mitted through the placental decidua is, and probably must remain, an unexplained 

 and unexplainable mystery. As is the case with all mysteries, the Intestinal Worms, 

 more especially those which inhabit the human viscera, have led to a great deal of 

 mystification and quackery, and nostrums innumerable are recommended to the public ; 

 nor are there wanting fabricated imitations of some of the more formidable species, 

 usually prepared from the intestines of other animals.] 



The Entozoa are true parasites, and cannot assimilate matter for their own growth 

 and nourishment unless they receive it from the body of a living animal. They have 

 no vestige of breathing apparatus, which shows that they must receive their nourish- 

 ment aerated by the breathing of the animals upon which they are parasitic. This 

 supersedes all necessity of a circulating system ; and the traces of a nervous one are 

 so very obscure that many naturalists have doubted its existence. When we find 

 the character and the form of these animals in any species, we include it along with 

 those which it most resembles, though it should not be parasitical within the body of 

 any other animal. The injury which these Intestinal Worms occasion to the animals 



