ECONOMIC REIvATIONS, ANATOMY, AND LIFE HISTORY OF GENUS LERN^A. 1 73 



of the (fused) first thorax segment soft horns, more or less branched, extend outward 

 and serve to anchor the parasite in the tissues of the host. The free thorax starts as a 

 slender neck, which increases gradually in diameter backwards, making the trunk of 

 the creature club-shaped. At the posterior end the dorsal portion of the thorax passes 

 insensibly into a short abdomen, which is much narrower than the thorax, bluntly 

 rounded, and tipped with a pair of tiny anal laminae. 



On the ventral side the thorax is abruptly narrowed to the diameter of the abdomen, 

 leaving a bluntly rounded protuberance for which Cunnington has proposed the name 

 of "pregenital prominence." 



The egg cases are attached to the ventral side of the thorax just in front of the 

 base of the abdomen; they are oval or eUiptical in outHne and much shorter than the 

 body. The eggs are large, spherical, and not flattened; they are multiseriate, but have 

 no definite arrangement. 



Torsion. — During the process of burrowing into the fish's tissues the body of the 

 parasite usually becomes twisted upon its longitudinal axis, so that the pregenital 

 prominence, instead of being on the ventral surface with the mouth and the appendages, 

 is turned to one side and may even be carried through 180°, so that it apparently lies 

 on the opposite or dorsal surface of the animal. 



Quidor has described this arrangement in a recent paper on some of the other 

 Lemseidse and has named it torsion. He called the torsion direct when it turns from 

 the left over toward the right, looking at the animal from the anterior end, and inverse 

 when it turns from the right over toward the left. He claimed that one kind of torsion 

 was shown when the parasite was attached to the left side of the fish and the other 

 kind when it was attached to the right side, and that the amount of torsion was constant 

 for a given species. The author has disproved this claim for other genera in the 

 Lemaeidae," and the few data available for this genus are all against it. 



The first adverse fact is the existence of altogether too many full-grown adults 

 without any torsion at all. An occasional specimen of this sort would only furnish the 

 usual exception that occurs with every rule. But in the data given below 8 specimens 

 of cruciaia out of 20 showed no visible torsion, namely, 40 per cent of the whole. 



Claus in deaUng with L. esocina noted that young forms 3.50 mm. long possessed 

 Straight bodies on which the four pairs of swimming legs were in the same straight line 

 and that the twisting took place gradually during subsequent growth'' Evidently 

 it frequently happens that there is no subsequent twisting, for every one of the eight 

 specimens just mentioned was 15 to 20 mm. long and carried egg strings. 



In the second place, specimens obtained from the same fish do not show the same 

 torsion. A single 9-inch largemouth black bass, Micropterus salmoides, was obtained 

 at Fairport, Iowa, which was infested with eight specimens of L. cruciata, two on the 

 right side, five on the left, and one on the median line behind the dorsal fin. Of the 

 first two one showed direct torsion of 180° and the other inverse torsion of 150°; the 

 single parasite on the median line showed an inverse torsion of 180°; of the remaining 

 five, twq showed direct torsion of 135° and 180°, respectively; and the other three 

 inverse torsion of 150°, 180°, and 180°. Thus neither the amount nor the kind of 

 torsion was constant, but it might be objected that the maximum difference is only 45° 

 and that five out of the eight specimens did show a torsion of exactly 180°. But even 

 three out of eight, 37 per cent, is still too large a number to be treated as mere excep- 

 tions, and this proportion is greatly increased in the following data. 



a Proc. U. S. Nat. Mus., vol. 53, p. 10. 



& Schrif t. Gesellsch. Beford. ges. Naturw. Marburg, vol. 9, supplement, p. 2, 1868. 



69571°— 18 -12 



