ENTOZOA. 
291 
metamorphosis, to which the larvae of Trematoda, hitherto known as 
Cercarice, are subject ; but he cannot agree with him in assuming, that 
the generation of the Cercarice is now completely understood ; for here, 
as will be afterwards pointed out, there are considerable gaps to be 
filled up. In no single species of Trematoda is this change completely 
known : here or there several links are wanting in the chain of the me- 
tamorphosis, which must be supplied by direct observation, before we 
can look upon the cycle of the different links of any one of them as 
concluded. The reporter has no doubt, but that these gaps will be filled 
up, especially as Steenstrup’s excellent idea points out a way to that 
object. In following it in order to develop these wonderful metamor- 
phoses of the Helminthes, we must not allow conjecture to slide into the 
series, and imperceptibly assume the place of approved fact. 
The early and much misunderstood theory of equivocal generation, 
particularly as regards the engendering of intestinal worms, is now 
beginning to lose its weight with English Physicians. Dr. Watson is 
inclined to the opinion, that the Helminthes are introduced, as germs or 
eggs, within the human body (London Med. Gaz., May, 184:2, part 2. 
vol. ii. p. 231) ; and he questions, whether some of the Entozoa may 
not be originally Ectozoa. Against the doctrine of equivocal generation 
he relates the following circumstance, told him by Abernethy: — A 
healthy flock of sheep were driven through a considerable tract of 
country, and one of them on the way broke its leg, and had to be car- 
ried on horseback. For one night the flock, with the exception of the 
maimed one, rested in a marshy meadow, and every individual was 
seized with the rot but itself; it escaped the disease, and had no liver- 
fluke. Watson asks, whether it may not be assumed, that the flock 
swallowed the eggs of the fluke with the fodder they crept from the 
moist meadow. The eggs might then, as is the case, especially with 
Helminthes, shut up in cavities, be conveyed by the blood to the liver. 
The reporter has often heard this opinion expressed by Physicians and 
Naturalists ; but he cannot conceive how the eggs of Helminthes, re- 
maining passive in the intestinal canal of an animal, should get into 
the vascular system, which is everywhere shut up from it. However, 
he is also perfectly convinced, that many Helminthes, after throwing otf 
the egg-covering, can pass, in their embryo state, through the paren- 
chyma, to the organ suitable to them. Hammerschmid made a valuable 
observation on the origin of Helminthes, at the fourth meeting of Italian 
Literati at Padua (Berliner Vossische Zeitung, 14th Oct. 1842), viz,, — 
that Tenehrio molitor is afflicted with intestinal worms when it feeds 
on flesh, and is free from them when it consumes meal. 
A remark of Dr. Wolfring may be mentioned, on the geographical 
distribution of the Helminthes (Medic. Corresp. Blatt bayerisch. Arzte, 
1842, p. 805). He described the district of Thalmessingen as a valley 
335 
