554 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.55. 



be accomplished chiefly by the mouth parts. There is also an elonga- 

 tion of the thorax accompanying the burrowing, but it differs from 

 that of the Lernaeids in one important particular. In the Ler- 

 naeidae, as is shown by the persistence of the swimming legs, close 

 together and just behind the head, the elongation is produced by 

 the fifth and sixth thoracic segments. The only exception is found in 

 the genera Peniculus and Lernaea^ in which the anterior segments of 

 the thorax take part in the elongation. In the former genus, Penicu- 

 lus^ the second and third segments are slightly elongated, so that the 

 four pairs of legs do not stand close together, but while the first and 

 second pairs are still in juxtaposition, the third and fourth pairs are 

 each removed to a little distance posteriorly. But even here the so- 

 called neck ends with the third segment, and the remaining segments 

 are fused into a common trunk. In Lernaea each thorax segment 

 contributes its share to the elongation, and each pair of legs is re- 

 moved from the one which precedes it by a distance which increases 

 as we proceed backward, bringing the fourth legs close to the posterior 

 end of the body. And there is often a fifth pair of rudimentary legs 

 just over the bases of the egg strings, in a position corresponding to 

 that found in the Caligidae. But there is no real neck, the anterior 

 thorax passing insensibly into the posterior, with no definite line of 

 demarkation. 



Here in the Sphyriidae also all the thorax segments take part in 

 the elongation, but in females like the one represented in figure 68, 

 on which portions of the swimming legs still persist, it can be seen 

 that each pair is separated from the one preceding it by a distance 

 which at first increases and then decreases as we proceed backward. 

 This means that the third and fourth segments contribute more than 

 the others to the elongation. Moreover, the position of the fourth 

 legs at the anterior end of the trunk shows that the latter is really a 

 fusion of the fourth, fifth, and sixth segments. 



Torsion. — With reference to torsion the same may be said that has 

 already been given in the case of the Lernaeidae.^ In burrowing 

 through the tissues of the host the head of the parasite does not 

 always come in contact with the dorsal aorta, and the parasite has 

 to burrow to the right or left in search of it, thereby producing a 

 twisting of the body around its longitudinal axis. If the burrowing 

 starts on the right side of the host, the head is more often turned to 

 the left in search of the aorta; if on the left side, it is more often 

 turned to the right; if on the median line behind the dorsal fin, it 

 may be turned in either direction to get around the vertebral column. 

 As a result we find varying degrees of direct or inverse torsion as in 

 the Lernaeidae, and similar torsion must be found in any parasite 



irroc. U. S. Nat. Mus., vol. 53, p. 10. 



