312 ANIMAL PARASITES. 



probably, contented with a passive migration from intestine to 

 intestine, and it is, perhaps, sufficient for this purpose that the 

 egg of a worm should pass out into the water, undergo segmenta- 

 tion, and form the embryo there, and that the embryo should 

 be swallowed with drink, and become directly further and more 

 highly developed in the intestine of its new host, which must 

 usually belong to the same species as the previous host ; but 

 in the armed species, on the other hand, a mixed, passive- 

 active-passive, migration will take place. YVe may, perhaps, 

 draw conclusions, per analogiam, as to the migration of the 

 armed brood of nematode worms, from what has been said 

 by Von Siebold with regard to the brood of Merrnis albicans, 

 and by Meissner, with regard to Gordius, and the destiny of the 

 brood of these animals. 



In the case of the so-called Filarice of insects, Yon Siebold 

 convinced himself these round-worms are no true FUarice, but 

 that they belong to the peculiar genera of round-worms, which 

 are usually called Gordius and Mermis. In their fully mature state 

 they wander from their previous dwelling-place, and perforate the 

 walls of the body of their host in any soft place, in the same way 

 as the larva of the horse bot-fly, which finally quits the stomach 

 and intestine of the horse, or in the manner of the larva of the 

 gadflies, which, at a certain time, bore their way out of the 

 cutaneous excrescences of cattle. A peculiar instinct impels 

 them, in the perfectly developed but still asexual state, to such an 

 emigration, in consequence of which they leave their previous 

 dwelling-place for a new phase of existence, which brings them to 

 their sexual developments, and we find them in the open water 

 or in moist spots in the ground, in digging up beds, or making 

 drains in the fields, or in drinking-water. Yon Siebold then 

 succeeded further in ascertaining how the immature but almost 

 completely developed worms taken out of the caterpillars of the 

 small ermine moth (Yponomeuta evonymelhi), immediately bur- 

 rowed with their heads into the moist earth of the flowerpot 

 upon which they were laid, and in the course of the winter 

 became sexually mature in this flowerpot, which was kept moist, 

 developing eggs in their interior, which were afterwards deposited 

 in the earth, to the number of many hundreds. In the first 

 days of spring the embryos were completely developed in the 

 interior of these eggs, and embryos, which had left their egg- 

 shells, were then soon found. When Yon Siebold had noticed 



