NEMATHELMTNTKRS 245 



now arises at one end. In this tlie head of the embryo is 

 formed in such a way that it can subsequently be everted. 

 The head is composed of a thicker basal portion and a slender 

 proboscis. The former bears three circles of six hooks each, 

 the latter three long stylets, so that the embryo appears well 

 armed. 'At the time of hatching, the head, with its armature, 

 is everted (Fig. Ill J5), but can at any time be retracted as 

 before (Fig. Ill ^). In the mean- 

 time the intestine is formed, and 

 leads from the mouth, at the tip of 

 the proboscis, to the anus, situated 

 somewhat in front of the posterior 

 end. The efferent duct of a remark- 

 ably extensive gland opens into the 

 oesophagus, at the base of the pro- 



-, . Fig. 111.— J and B, two 



boscis. Externally the embryo pre- larva? of Gorduts subbi/ui-cus, 

 sents an annulated appearance (Fig. ^^^^ retracted and with 



, , 1 X rm 1 p, 1 i 1 • everted proboscis (after 



ill;. Ihe embryo alter hatching meissnbe). 

 lives as a larva for a long time free 



in the water, and then, with the aid of its sharp armature, 

 penetrates through the skin into the interior of Chironomus 

 larvas, as was observed by Villot. This observer regards 

 as simply an exceptional case Meissner's statement (No. 13) 

 that the larvae of Ephemeridss are also infected with Gordius 

 larvae. The parasite becomes sui'rounded by an envelope 

 formed from the tissue of the Chironomus larva, and re- 

 mains there until the larva happens to be swallowed by a 

 fish (Villot, No. 16). Becoming free in its intestine, the 

 Gordius larva perforates the intestinal wall and again be- 

 comes encysted. It remains here for a long time without 

 undergoing essential change. Finally (at the beginning of 

 spring) it returns to the intestine, which it leaves with the 

 faeces, and then gradually assumes the form of the adult 

 worm, during which the cephalic armature is lost, the an- 

 nulation of the surface of the body is smoothed out, and the 

 sexual organs are developed. At the same time its intestine 

 suffers a partial degeneration, and the mouth becomes closed. 

 However, it appears to be by no means certain whether 

 the mode of development described is the one realized, or 



