IV.] INTRODUCTION. Ixi 



cavity at pleasure ; and some which I saw retire into it 

 never reappeared, although I watched the slug narrowly 

 for a considerable time. It is remarkable that the slug 

 appears to suffer no particular inconvenience from these 

 parasites, and even allows them to run in and out of the 

 lateral orifice without betraying the slightest symptoms 

 of irritation." M. Barthelemy has noticed in the eggs of 

 one kind of slug numbers of a small Nematoid worm, 

 which he has named Ascarioides limacis. These worms 

 are present in the egg at the moment of its being laid, 

 having been previously deposited by the parent worm 

 while living in the ovary of the slug. The young worms 

 must therefore have been introduced into the egg while 

 it was being formed. They appear to adopt the same 

 course as the parasitic larvae of the Ichneumon when 

 they are deposited in the body of a grub, and to spare the 

 vital parts of the embryo on which they feed, until the 

 period has arrived for their own development. In fresh 

 water, the pond-mussel (Anodonta) constantly entertains 

 a large party of parasites, consisting of another kind of 

 mite (Atax ypsilophora, Buntz), which are so tenacious of 

 life, that after their host has been put into boiling water 

 and killed they survive and crawl about as if nothing 

 had happened to them. A kind of hair-worm (Gordius 

 inquilinus, Miiller) attaches itself in clusters to Limncea 

 stagnalis and many other freshwater snails. In all pro- 

 bability, however, these are not true parasites, and only 

 attach themselves to the Mollusca for the sake of the 

 shelter afforded by their shells or mantles, obtaining 

 their food from the water and not from the snails, be- 

 cause their heads are always seen outside and in active 

 motion when the snails are crawling. Each genus, if 

 not every species, of freshwater snail may have its own 

 pseudoparasite. Limncea, Planorbis, Physa, Ancylus, 



