THE CRUSTACEA 



distal segments winch are present in the larva persisting as a 

 vestige under the outer margin of the sucker or disappearing 

 altogether. The sucker itself is provided with strong muscles and 

 its margin is strengthened by radiating chitinous rods. 



The first thoracic appendage ("second maxilliped") (mxp) consists 

 of five segments, the basal one produced into a lobe armed with 

 strong teeth and the end of the limb carrying two stout claws. 



The next four pairs of appendages are biramous natatory feet. 

 The first pair is attached to the cephalothoracic region, the remain- 

 ing three correspond to the three free thoracic somites. Each 

 consists of a stout protopodite, which, in some species at least, has 



sp 



Argulus americanus, female, from below, x 5. a', antennule ; a", antenna ; e, paired eye ; 

 /, caudal furca ; mx", sucker formed by maxilla (" first maxilliped ") ; mxp, first thoracic 

 appendage (" second maxilliped ") ; p, suctorial proboscis ; sp, poison spine. (After Wilson.) 



three segments, and an exopodite and endopodite of about equal 

 length. The exopodite is unsegmented, and the endopodite consists 

 in the first pair of three segments, in the second of one, and in 

 the last two pairs of two segments. Both rami are furnished with 

 two rows of long plumose setae, set along the dorsal and ventral 

 margins respectively. The insertion of the broadened bases of 

 these setae gives to the exopodite and endopodite the appearance of 

 being divided into numerous short segments, and they have been 

 so described, but the arrangement of the musculature shows that 

 these do not represent true segments of the limb. 



On certain of the anterior legs in most of the species the 

 protopodite carries at its distal end, besides the endopodite and 

 exopodite, a slender appendage, known as the flageUum, which 



