TETEARHYNCHIDiE. 135 



witMn a short distance of the commencement of the caudal vesicle, 

 at which point they severally terminate in club-shaped enlarge- 

 ments. Each proboscis itself appeared to be tubular, being con- 

 nected to the club-shaped pouch by a special muscle (retractor 

 prohoscidis; fig. 8). The walls of the pouch are very thick, and 

 from their muscular character seem to be the principal source of 

 the protrusive power of the armed proboscides. In point of fact, 

 the whole apparatus may be compared to a mihtary weapon, in 

 which we have a long barrel formed by the muscular sheath, an 

 orifice comparable to the muzzle, an armed rod forming the pro- 

 jectile, a special muscle forming a connecting cord, and a strong 

 breech as a point d'appui for the development of the propelhng 

 force. In some tetrarhynchs these pouches present a more oval or 

 rounded figure, and there is every possible variety in the length and 

 caliber of the sheath and its contained proboscis. The same varia- 

 tions are noticeable as regards the armature of hooks for boring. 

 In the present case each proboscis is supphed with two or three 

 hundred circular rows of hooks, four to each row, but the size 

 and configuration of the hooks are by no means uniform. There 

 are, in fact, four difierent sizes, which are repeated at regular 

 intervals, each group of sixteen occupying, as it were, a distinct 

 section or segment of the proboscis. Of these four circles, the 

 uppermost of the series is disproportionately developed. Altogether 

 there must be several thousand hooks. These organs are employed 

 as instruments for boring through the flesh and tissues of the 

 host ; and from their peculiar construction they may be looked 

 upon as four chain saws, whose efficiency is very greatly increased 

 by the circumstance that each chain presents four serrated edges, the 

 teeth of which are of four difierent sizes. Not a few Tetrarhynchi 

 exhibit even a still more complex armature, as may be seen in the 

 Tetrarhynchus longicoUis infesting the tope or penny-dogfish (Galeus 

 vulga/ris).* It maybe difficult in some instances to understand how 



* other forms are figured and described in Van Beneden's earlier quarto work, " Les 

 Vers Oestoides," p. 151, et seq., Plates xv. — xx. 



