218 DADD’S VETERINARY MED{CINL AND SURGERY. 
a round worm, called ths strongylus gigas, measuring from five 
inches to three feet in leugth, and from two to six lines in thick- 
ness. 
The diseases with which even large intestinal worms are con- 
nected appear to be sometimes the cause and sometimes the cffect 
of the presence of these parasites. Very often they exist in con- 
tierable numbers without producing the least disturbance of the 
eccnomy, but in other cases they are unquestionably the cause of 
much suffering and ill-health. How far they are themselves the 
result of a morbid state of the organs in which they appear is 
still an undecided question. 
The origin of parasites is extremely obscure, and has long been 
@ mooted point among naturalists. It may not be inappropriate 
to present a summary of the opinions which are entertained respect- 
ing a subject of so much interest, but, in doing so, we shall confine 
our remarks to the parasitic animals which inhabit the interior of 
the body, or entozoa. 
It is evident that these animals must originate in one of two 
ways; that they must be derived directly or indirectly from with- 
cat, or be created out of materials existing within, and furnished 
by, the body. No other supposition is possible. If an entozodn 
is in any manner derived from without, it must be admitted that 
this takes place either through the reception of the animal itself 
or of its ova. If either opinion be assumed, it follows that the 
parent animal must exist somewhere external to the body. But 
the parasites in question have never, in any case whatever, been 
detected except within the organism. Ifit is objected that many 
of these animals are so minute that they might easily elude dis- 
ecvery in the elements around us, the argument fails when applied 
to the giant strougylus, the stout lumbricoid worm, and the tenia, 
measuring many yards in length. Besides, even admitting for a 
moment the possibility of the parasites which inhabit the intes- 
tine, and other mucous cavities, having once existed externally, 
the insuperable difficulty still remains of explaining the entrance 
of entozoa into shut cavities and parenchymatous structures—into 
the eye, or the muscles, for example, and their presence in the 
unborn child, and even in the bodies of larger entozoa of a differ- 
ent species. On the other hand, if it is maintained that the ova 
are alone received, it must still be shown that the ova exist exter- 
nal to the body, which has never been done. Nor would the 
