632 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. XXVII. 



Fig. 6. — Posterior maxillipeds argulis americanus. 



are two concentric rows of the rods, the inner about twice the length of 



the outer, while outside of the latter is a wide free marg-in (fig. 5). 

 The rods in })oth rows are shaped like the letter J when viewed from 



the inside, the bases of 

 those in the outer row 

 articulating with the tips 

 of those in the inner row. 

 Around the edge of the 

 membrane is a fringe of 

 elongated linger-like pa- 

 pilla^, each terminated b}^ 

 three or four slender hairs. 

 Posterior niaxillipeds (tig. G). — These are small but well armed. Of 



the tire joints, the three terminal ones are much smaller than the two 



basal ones. The basal plate has an elongated triangular form and 



carries upon its posterior margin three 



teeth so wide and so squarely truncated 



as to be plates rather than teeth, as is 



also the case in A. catostomi. The plates 



are reenforced by a pair of stout, blunt 



spines near the median line just behind 



the mouth. The papillated area is small, 



but has a bunch of long and sharp setse 



on its posterior margin; the remainder 



of its surface and the entire ventral sur- 

 face of the four terminal joints are cov- 

 ered Avith short, stiff hairs, each with a 



swollen base. 



The terminal joint ends in three small, 



rather blunt claws of about the same size, 



arranged in a broken row anteriorly, 



while opposed to them posteriorly is a 



short, rounded papilla or thumb. 



Sioimnnng legs. — These are long and 



slender, projecting well beyond the edge 



of the carapace, the two anterior pairs with 



recurved tlagella. The basal lobes on the 



posterior pair are large and boot-shaped, 



with a distinct heel. The}^ are fringed 



along the posterior margin with a row of 



plumose setfB, and the toe of the boot car- 

 ries two much longer setjs, also plumose. 



Tactile fapillae. — These, for ovipositing, are large, long, and quite 



widely flaring in the female, but reduced to mere stumps in the male. 

 Circulation. — This, while agreeing with that in other species, differs 



markedly in the structure and working of the heart (fig. 7). This is 



Fk;. 7. — Dorsal view of the heart 

 of argulus americanl's (diagram- 

 matic), o, paired lateral open- 

 i.N'(;s; h. ANTERIOR opening of aorta; 



C, ANTERIOR: d, POSTERIOR VENTRAL 

 OPENING. 



