OBSERVATIONS ON THE MIGRATION OF FREE EMBRYOS. 63 



parasites and their hosts, and by subsequently finding the parasites 

 within the bodies of the latter, which of course had been previously 

 ascertained to be free from parasites. Von Siebold, 1 in the account of 

 his researches into the Mermithidae,and their wandering into the bodies 

 of minute caterpillars, makes the following remarks : " Thirteen larvae 

 of the spindle-tree moth (Hypomeneuta cognatella), which I had 

 previously found by microscopical examination to be free from 

 thread-worms, were placed in a watch-glass, in which was a quantity 

 of damp earth containing active embryos of Mermis. After eighteen 

 hours, I was able to detect these embryos in five of the caterpillars. 

 In a second experiment, I carefully examined thirty-three caterpillars, 

 to see that there were no Nematode larvae in them to start with, and 

 placed them in similar conditions. After the lapse of twenty- four 

 hours, fourteen of them contained embryos of Mermis, six of them 

 contained two worms a piece, two others contained as many as three 

 a piece. I also made use of young caterpillars not more than three 

 lines in length of Pontea crateegi, Liparis chrysorhcea, Gastropacha 

 neustria, which I took out of the webs in which they had hibernated. 

 They were in a similar fashion placed in a watch-glass with damp earth 

 and embryos of Mermis. On the following day I found that ten out 

 of the fourteen contained embryos ; in five there were two larvae, and 

 in one there were no fewer than three." 



Meissner 2 has recorded similar observations upon the embryos of 

 Gordius. The wandering into the bodies of larvae of Ephemera, which 

 Meissner made use of for his experiments, 3 only took place at night, 

 and always through the appendage which served as a point of attach- 

 ment for the young larvae. " All the Ephemerid larvae which were 

 left for the night in a vessel with the Gordius-embYjos were attacked 

 by them ; all the intruders, however, were found in the legs, usually 

 in the neighbourhood of the first joint, but some had penetrated 

 as far as the muscles of the coxa ; some were quiescent, with the head 

 and proboscis retracted, but the majority were actually moving about, 

 and I was able to see them in the act of making their way between 

 the muscle-bundles. This was done in a very peculiar way. The 

 head was thrust forward, and the hooks, being directed outwards, 

 obtained a firm hold of the tissues ; the head and proboscis were then 

 drawn back, to be again thrust forward in the same way. The pro- 

 boscis thus penetrated some distance, and the hole was then enlarged by 

 the head with its circle of hooks. The contractions of the muscles of 



1 Entomol. Zeitung, p. 239, 1860. 



2 Zeitschr. f. wiss. Zool., Bd. vii., p. 132, 1856. 



3 Villot considers that the larvae of Chironomus, and not Ephemera, are the proper 

 hosts of the young Gordius. Archives d. Zool. exptr., t. iii., p. 186, 1874. 



