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 internal 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 som: of the more important and useful of the reveat gen- 
eral works on parasitic worms: : 
Félix Dujardin, Histoire naturelle des. Helminthes au vers intestinaux, 8vo, 
with plates, Paris, 1845. (Suites a Buffon.) 
C. M. Diesing, Systema Helminthum. Two volumes, 8vo. Vindobone, 1850. 
J. Leidy. A Flora and Fauna within living animals. Smithsonian contribu 
tions, 1852. 
C. Davaine, Traité des Entozoaires et des mala‘lies vermineuscs de ’homme 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 ; andmtreduptiantesthi@study of Helminthology, with 
