802 Description of the Argulus Catostomi. 



flattened on the under side, which is also papillose ; the fifth is simi- 

 lar to the fourth, and about one third its length ; the terminal is 

 provided with two apical hooks. 



The natatory or branchial legs arise in a series on each side of the 

 abdomen. The three anterior pairs are composed each of three, the 

 fourth of tivo, large fleshy joints, and are terminated by two long pin- 

 nulae. Along the posterior edge of the second and third joints of the 

 two anterior natatories, is a ciliated ridge ; a corresponding ridge is 

 observed on the third joint of the third pair, while on the second joint 

 there is substituted a ciliated lamina : similar laminae are situated on 

 both joints of the fourth pair, which on the basal joint is large and 

 cultriform, and covers the termination of the abdomen. The edges 

 of the pinnulse are provided each with a row of transparent plumose 

 ciliae. These rows are inclined to one another at an angle of about 

 120°, and in the usual position of the pinnulee, one is invisible, it 

 being directed towards the shell. 



The outer pinnula of the first pair of natatories is three-jointed ; 

 (Fig. 7.) the first joint occupies nearly its whole length, the other 

 two are very short and destitute of cilise : at the apex are two mi- 

 nute setae. Along the centre of these pinnules runs a dark vessel, 

 which is probably connected with the branchial cihae. At the base 

 of the pinnulae of the first and second pairs of legs on the upper side, 

 arises a recurved pinnula, composed of two nearly equal joints, and 

 ciliated like those above described.* During life, the legs are ex- 

 i;ended a little fpr-ward, and the pinnulae are wholly covered by the 

 shell. At death they are inclined backward, as in Fig. 9. 



The abdomen is somewhat depressed and composed of four joints, 

 each of which gives rise to a pair of natatory legs ; the fourth joint 

 extends mostly beyond the shell. From its extremity proceeds a 

 broad rounded lamina, bilobate posteriorly, and provided with two 

 minute projecting ciliated plates, at the base of the terminal sinus. 

 This caudal lamina or tail has an entire and diaphanous margin, and 

 is destitute of cilise. 



Extending from the termination of the abdomen, and partially cov- 

 ered by the cultriform plates on the fourth pair of natatories, are tw6 

 narrow laminae, (Fig. 1. ss.) near the base of which are the organs 



* In the Argulus foliaceus, the first outer pinnula is not stated to be articulated ; 

 neither is mention made of any joints in the fleshy part of the natatory legs, or in 

 the recurved pinnulse. 



