82 Part III. — Twenty-sixth Annual Report 



rack and Oresund * and Johnston from similar Dog-fishes and from Thorn- 

 back Skates, trawled in the Irish Sea.f 



Tetrarhynchus minutus, P. J. van Beneden. PI. V., figs. 7-8. 



1850. Tetrarhynclius minutus, van Ben., Les Vers Cestodes, 

 p. 157, PL XX. 



This Oestode was obtained in the intestine of an Angel-fish, Squatina 

 angelus, captured in the Firth of Clyde in May 1904 ; it is a small species 

 and easily overlooked. As indicated above, this species was described by 

 van Beneden in 1850, and the characters by which he distinguishes it are 

 as follows : — " Les bothridies ne sont pas completement separees les unes 

 des autres ; les trompes sont couvertes de crochets recourbes ; les gaines 

 des trompes forment des tours de spire ; les segments sont tres-longs et peu 

 nonibreux," and he adds that the species may be recognised from closely- 

 allied forms by its small size, the length of the segments, which are several 

 times longer than broad, and the number of articulations, which seldom 

 exceed six, the last segment being already mature when five or six rings 

 can be counted, whereas in other species mature segments do not usually 

 occur till a larger number of rings have been formed. Van Beneden's 

 specimens of T. minutus were also obtained from Squatina angelus, 

 which appears to be the only kind of fish this Oestode has been recorded 

 from. 



Another species of Tetrarhynchus — T. erinaceus, P. J. Van Beneden, 

 described in 1858,$ has been noticed in fishes examined at the Labora- 

 tory, usually in small cysts on the walls of the stomach, and pyloric caeca 

 of Gadoids (Cod-fishes and Saithe). T. erinaceus, in this encysted state, 

 according to van Beneden, is unable to attain sexual maturity, and is 

 therefore placed by him among the zenosites or strangers — parasites that 

 have not yet reached their ultimate destination, or, as that author remarks, 

 " Ce sont des parasites en transit." The encysted Tetrarhynchus can only 

 reach the sexually mature stage after it has been transferred to the 

 stomach of some Plagiostome, and the fish belonging to that group in 

 which the parasite has been most frequently observed in a sexually 

 mature condition is the Thornback Skate, Raia clavata. The proboscides 

 do not appear to be exserted while the parasite remains within its cyst, 

 but when removed from it and placed in a little sea-water the Cestode, 

 apparently recognising the change in its environment, aoon begins to push 

 out its formidably armed proboscides. So far as I have observed, the 

 thrusting out of these armed appendages is not completed by a continuous 

 movement, but intermittently, as if the operation were a work of some 

 difficulty, and that a pause was necessary for further effort. I have also 

 observed that, though the fish may have been dead for a good while, the 

 encysted parasite would be still alive, and on being removed from its 

 prison would in a short time begin to thrust out its proboscides. 

 T. erinaceus is a widely-distributed species, either in its encysted state or 

 in its state of sexual- maturity, for it has been recorded not only by van 

 Beneden, Olsson, and other European Helminthologists, but also by Linton 

 in his papers on the Eutozoa of American fishes. 



Two of the species of Tetrarhynchus mentioned here — T. tetrahothrius 

 and T. mimitus — have also been assigned to the genus Rhynchohothrium, 

 Rudolphi, but meanwhile I leave them where van Beneden placed them. 



*Bidrag till Scandinaviens fauna, Kongl. Sv. vet. Akad. Handl., Bd. 25, 

 No. 12, p. 25 (1893). 

 tRept, for 1905 of the Lancashire Sea Fisheries Laboratory, p. 174 (1906). 

 I Mem. sur les Vers intestineaux, p. 128, PI. XVIII. 



