52 BOARD OF AGRICULTURE. 



of their worst parasites. Thus some of these creatures are 

 kept in existence solely by reason of the peculiar relations ex- 

 isting between man and his brute dependents. This is partic- 

 ularly the case in respect to the two most common tape-worms 

 of man, derived respectively from the flesh of cattle and hogs, 

 in which the young worms live. These two worms appear to 

 be capable of coming to maturity only in the human in- 

 testine. 



Most of these internal parasites belong to the great class of 

 articulated animals known as Worms, constituting, however, 

 several distinct orders, which are often collectively called 

 Helminths. The orders of parasitic worms are much lower 

 in rank than those that include the independent kinds, like 

 the numerous species of free marine worms, or Annelids, 

 many of which have beautiful gills and locomotive organs, a 

 very complex circulation, red blood, and a well-organized head, 

 eyes, mouth, and intestine ; to such worms the common earth- 

 worms, or angler-worms, are also allied, although destitute of 

 gills. But the parasitic worms never have red blood nor 

 special organs for breathing, and rarely for locomotion ; most 

 have no distinct organs of sense, and many have no head and 

 no intestine. The reproductive system is, however, always 

 very highly developed, and most of them are exceedingly 

 prolific. This is essential to their existence, for it is often only 

 by mere chance that any of the eggs or young reach the 

 peculiar habitats in which alone they can grow to maturity. 



The int<irnal parasites of animals have long been the sub- 

 ject of laborious and careful research by many of the ablest 

 naturalists and physiologists,* owing to the remarkable 



*The following are some of the more important and useful of the recent gen- 

 eral works on parasitic worms : 



Felix Dajardin, Histoire naturelle des Helminthes au vers intestinaux, 8vo, 

 with plates, Paris, 1845. (Suites a Buffon.) 



C. M. Diesing, Systema Helminthum. Two volumes, 8vo. Vindobonae, 1850. 



J. Leidy. A Flora and Fauna within living animals. Smithsonian contribu 

 tions, 1852. 



C. Davaine, Traite" des Entozoaires et des mala-lies vermineuses de rhomme et 

 des animaux domestiques, 8vo, 930 pages, with 88 figures. Paris, 1860. 



D. F. Weinland. An Essay on the tape-worms of man ; 8vo pamphlet, with 

 figures. Cambridge, Mass., 1858. 



T. S. Cobbold. Entozoa ; an Introduction to the study of Helminthology, with 



