182 



NEMATHELMINTHES 



present differences which enabled him to divide the group into 

 three families, each with a corresponding genus To these I 

 have ventured to add a fourth family, to include a remarkable 

 species, Arhynchus hemignathi, described below. The char- 

 acters of the first three families in the account given below are 

 taken from Hamann's paper. 



Family I. Echinorhynchidae. The body is elongated and 

 smooth. The proboscis-sheath has a double wall, and the pro- 

 boscis is invaginated into it. The 

 central nerve -ganglion lies in the 

 middle line, as a rule on the pos- 

 terior blind end of the proboscis- 

 sheath. The papillae which bear the 

 hooks are only covered with a chitin- 

 ous cap at their apex, and the hooks 

 have a process below. This family 

 is by far the largest ; a few species 

 only can be mentioned. EcJiinorhyn- 

 chus proteus lives in its mature form 

 in fishes ; the young forms, up to a 

 centimetre in length, are found living 

 freely in the intestine of numerous 

 fresh-water fishes. Those found in 

 Gobio fluviatilis, the gudgeon ; Leu- 

 ciscus virgo; Lota vulgaris, the burbot 

 or eel-pout ; young trout ; Tliymallus 

 no. lOO.-Fully formed larva of vulgaris, the grayling, seldom surpass 

 Echinorhynchus proteus from this size, but those found in Acerina 



the body -cavity of Phoxinus ,-, ~ , A -, 



laevis. (From Hamann.) Highly cernua, the pope fish ; in Abramis 

 magnified, a, Proboscis ; b, Uvunctatus ; in Esox lucius, the pike, 



bulla ; c, neck ; d, trunk ; e, , . ,, 



e, lemnisci. and in older trout, attain or surpass 



double the length. As the parasites 



grow older they bury their proboscis and neck in the wall of the 

 intestine, the inner surface of which is studded with the orange- 

 coloured bodies of the parasites. The proboscis is so deeply 

 sunk in the wall of the alimentary canal as to form a papilla 

 on its outer surface (Fig. 92). The larvae of E, proteus are 

 found in the body-cavity of Gammartts pulex, one of the Amphi- 

 pod Crustacea, and also in the same position in numerous fresh- 

 water fishes ; they must have passed into this first host by the 



