656 ENTOZOA. 



Fishes, particularly the Skate. Leeches are so much used for 

 medicinal purposes that methods have been adopted for cultivat 

 ing them. Some enterprising Frenchmen have recently leased 

 marshes in Ireland, and sowed them broad cast with leeches, in 

 the hope of thus deriving large profits. The value of those 

 annually used in France, is estimated at from one to one and a 

 half million of dollars. The species H. geobdella, (Gr. earth- 

 leech,) frequently leaves the water to pursue earth-worms. 



FOURTH ORDER. ENTOZOA, (Gr. svrog y entos, within; t&ov, 

 zdon, an animal.] 



This order includes the various minute animals which are 

 produced and developed within other living beings. They are 

 exceedingly various in form and organization, having but one 

 character in which they mostly agree, viz. : that they are para 

 sitic, living within and at the expense of the bodies of other ani 

 mals. 



Some species, both in their appearance and internal structure, 

 so closely resemble individuals placed in other classes, that they 

 can be said to differ from them only in respect to the localities in 

 which they are found. They have been discovered in all the 

 Mammalia, from man down to the Cetacea ; and they are even 

 more numerous in Birds, Reptiles, and Fishes than in the Mam 

 mals. The invertebrated animals have also parasites peculiar 

 to themselves. They have been found in Insects, Mollusks, and 

 even the Acalephs. They fix themselves, according to the spe 

 cies, in various parts of the bodies which they infest, such as the 

 intestines, brain, liver, kidneys, muscles, blood, and bones. In 

 some cases, the same species are found in water, as well as 

 within animals. 



FIRST SUB-ORDER. NEMATOIDEA, (Gr. y^, nema, a thread ; 

 eidog, eidos, form.) ROUND WORMS. 



These are the highest in organization of the Entozoa, having a 

 round, long, and elastic body, and a complicated structure, there 

 being a true intestinal canal. The mouth, by its varieties, af 

 fords generic characters ; the females are longer than the males, 

 and for the most part oviparous. They have been divided into 

 eleven genera. We have room to notice only (1) those of the 

 genus Ascaris, which include the COMMON ROUND WORM, A. 

 lumbricoides, so named from its general likeness to the Lumbri- 

 cus, or Earth- Worm. This occurs in the hog and ox as well as 



