FISHES AND MANKIND 419 



as Diphyllobothrium latum passes the last larval stage of its develop- 

 ment in fishes, the larvae occurring in large numbers in the 

 viscera and muscles of the Pike and other fresh-water species, 

 which become infected by swallowing the small "water-fleas" 

 in which the earlier stages occur. Cooking destroys them, but 

 in parts of Eastern and Central Europe, as well as in North 

 America and Japan, the fish are eaten in a raw, smoked, or 

 inadequately cooked state, and the worm continues its life-cycle 

 in the intestine of its human host. The larvae (plerocercoids) 

 may grow to a length of twenty to thirty feet, and are sometimes 

 responsible for a severe type of anaemia. Man is not the only 

 host, the same worm occurring in wild and domesticated 

 carnivorous mammals. 



Round Worms or Thread Worms {Nematoda) of various kinds 

 occur in the adult form in almost all fishes, generally in the 

 alimentary canal, while larval Nematodes may be found in 

 the connective tissue, body cavity, or muscles. These parasites 

 rarely do much harm unless very abundant. Small Nematodes 

 are frequently to be seen encysted or free in the tissues sur- 

 rounding the abdominal cavity of such food-fishes as the 

 Cod, Hake and Haddock, and many people hesitate to eat the 

 fish on this account. Such fears are quite groundless, how- 

 ever, for not only are the worms destroyed by cooking, 

 but they are not of the type likely to flourish in a human host. 

 Thorn-headed Worms [Acanthocephala) may occur in large 

 numbers in the intestines of fishes, where they may sometimes 

 cause intense irritation and gastric disturbance. The larval 

 stages are passed in some smaller animal, forming part of the 

 normal diet of the fish. 



Monstrosities are comparatively rare in a state of nature, and 

 cannot be considered in any detail. "Bulldog-nosed" or 

 "Pug-headed" Trout, Pike, etc., in which the snout is abnor- 

 mally shortened so that the lower jaw projects, are captured 

 from time to time, and hump-backed or hog-backed specimens, 

 with the vertebral column shortened, curved, or otherwise 

 malformed, also occur among marine and fresh-water fishes. 

 Such individuals do not appear to be necessarily unduly handi- 

 capped in the struggle for existence, often attaining to a fair 

 age, and generally appearing well nourished. Abnormalities 

 of the fins also occur in a wild state, extra fins being developed 

 in unusual situations, or normal ones reduced in size or absent 

 altogether, as well as variations in scaling, coloration, and so 

 on. Among domesticated fishes monstrosities are much more 



