16 PROCEEDINGIS OF THE NATIONAL MUSEUM. vol.53. 



Causes which produce the torsion. — Quidor stated that " This tor- 

 sion is the result of the mode of fixation of the parasite, of the habits 

 of the host, and of the mechanical action of the external medium " 

 (1912c, p. 87). ^ 



AVe can readily understand how the resistance offered by the water 

 produces both flexion and torsion in the body of the parasite, and 

 how the amount of each might be varied by the habits of the host. 

 But neither of these causes explains the differences enumerated in 

 the above tables, especially the different kinds and amounts of tor- 

 sion in specimens attached at the same point and on the same host. 

 Again we are forced to the conclusion that there must be another 

 cause, stronger than an}' of those already mentioned. Quidor did 

 not explain what he meant by the " mode of fixation of the parasite," 

 but rightly interpreted this evidently becomes the controlling cause, 

 and is worthy of careful consideration. 



First comes the choice of a location on the body of the host; some 

 genera choose tlie gills or gill chamber, while others select some place 

 on the outside of the fish's body, a few species even preferring the eye. 



Attention has been elsewhere called to the burrowing of the para- 

 site into the tissues of the host, which varies greatly according to 

 the locality selected. In the throat it is only a short distance from 

 the outside surface or from the gill arches to the ventral aorta or to 

 the heart, and it is an easy matter for the parasite to find those blood 

 vessels. 



From the back or the sides of the fish it is a much greater distance, 

 varying with the size of the fish, and the parasite, after burrowing 

 through the tissues, will be fortunate if it strikes at once the blood 

 vessel that it started for. Then, too, the knowledge of the fish's 

 anatomy possessed by the parasite is not infallible, and it frequently 

 happens that the burrow strikes a rib or the backbone, around which 

 it must turn before reaching the desired goal. In such a case the 

 parasite may have to hunt around for some little distance before it 

 finds the blood suppl3\ This turning aside of the anterior end of 

 the body and the adjusting of the mouth to the blood supply is what 

 produces the torsion. Jungerson told (1911, p. 7) of a J'oung Sorco- 

 tretes scopeli fixed on the back of its host on a line with the root of 

 the pectoral fin. "It went nearly straight down through the mus- 

 cles, curved in front of the right pronephros over the upper pharyn- 

 geal bones, and had its anterior end lying in front of the left prone- 

 phros, with the sucking apparatus close to the jugular vein." Such 

 a twisting of the anterior part of the body would have produced 

 torsion even if there had been no external resistance. 



Similarly, Baudouin described the mode of fixation of Lernaeent- 

 cus sardmae, in the back of the sardine near the dorsal fin: "The 

 cephalothorax buries itself almost perpendicularly to the axis of 



