250 A HISTORY OF FISHES 



at the expense of the living tissues of its host. In man, the louse 

 provides a good example of an ectoparasite, while the tape-worm 

 is a well-known endoparasite. As might be expected, such a 

 mode of life is nearly always accompanied by some degree of 

 degeneration on the part of the tenant, and throughout the 

 whole of the animal kingdom internal parasites are, with few 

 exceptions, degenerate creatures, and in extreme cases only 

 the reproductive organs retain any semblance of their original 

 perfection. Cases of true parasitism among fishes are very rare, 

 although the Lampreys {Petromyzonidae) and Hag-fishes {Myxin- 

 idae) may be regarded as ectoparasitic. Certain members of a 

 family of South American Cat-fishes (Pygidiidae) are in the habit 

 of attaching themselves to any kind of fish or animal, piercing 

 the skin and gorging themselves on the blood of the living 

 victim. Others, known to the natives as "Candiru" or "Car- 

 nero" {Vandellia), habitually five within the gill-cavities of 

 large Cat-fishes (Sorubim, Platystoma, etc.), and other fresh-water 

 fishes, their slender form enabhng them to penetrate between 

 the gills, the sharp teeth and opercular spines being used to 

 start a flow of blood from the host, which is sucked up by 

 the mouth (Fig. gGv). The patches of spines on the gill-covers 

 also serve to assist the fish to wriggle between the gill-lamellae, 

 and to retain their hold when once estabUshed. In some parts 

 of Brazil the Candiru are very much dreaded by the natives, 

 owing to their unpleasant habit of entering the urethra of persons 

 bathing in the rivers, and both men and women are in the habit 

 of wearing special sheaths made of palm fibres to protect the 

 external genitalia, when obUged to enter the water. The Httle 

 fish appears to penetrate into the urethra especially, if not 

 always, during micturition, and it has been suggested that it is 

 definitely attracted by urine. It seems more probable, however, 

 that the flow of urine is merely mistaken by the fish for the 

 respiratory current coming from the gill-opening of a fish. 

 An accident of this nature may have serious consequences, for 

 once the fish has entered it cannot always be pulled out on 

 account of the erectile opercular spines, and a prompt surgical 

 operation is necessary to prevent it from reaching the bladder 

 and causing death from inflammation. Dr. Bach, who has 

 travelled extensively in the Jurua district of the Amazon system, 

 records that he examined natives — a man and three boys — in 

 which the penis had been amputated as a result of this dreadful 

 accident. 

 The only other case of parasitism occurs among the oceanic 



