302 Description of the Argulus Catostomt. 
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 two, large fleshy joints, and are terminated by two long pin- 
nulz. 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 lamine 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 pinnulz are provided each with a row of transparent plumose 
cilie. These rows are inclined to one another at an angle of about 
120°, and in the usual position of the pinnule, 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 cilie : at the apex are two mi- 
nute sete. Along the centre of these pinnule runs a dark vessel, 
which is probably connected with the branchial cilie. At the base 
of the pinnule 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- 
tended a little forward, and the pinnule 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 ciliz. 
Extending from the termination of the abdomen, and partially cov- 
ered by the cultriform plates on the fourth pair of natatories, are two 
narrow lamin, (Fig. 1. ss.) near the base of which are the organs 
* In the Argulus foliaceus, the first outer pinnula is not stated to be hashed 
neither is mention made of any joints in the fleshy part of the natatory legs, 0 
recurved pinnule. i 
