680 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



with blood sinuses and carrying" in their thickened bases the testes in 

 the male and the seminal receptacles in the female. Through the cen- 

 ter runs the much-narrowed cloacal portion of the intestine and around 

 this are longitudinal muscles which assist in circulation (p. 6()9). 



The outline of the -abdomen is usually oval or broadly triangular. 

 In some species whose cephalothorax is orbicular the al)domen also 

 approaches closely to that shape {catostomi). In other species {lati- 

 cauda, etc.) it is broadened posteriori}^ until it becomes almost per- 

 fectly elliptical. Its size and shape differ greatly in the two sexes even 

 of the same species. The presence of the long testes elongates the 

 lobes until sometimes the abdomen of the male is nearly twice as long 

 as that of the female {funduH). 



It is always much longer, and thus produces a narrowed elliptical 

 form, with a tendency toward acumination in the lobes posteriorly. 

 On the other hand, the spherical seminal receptacles of the female are 

 situated far forward in the lolies and tend to widen that portion espe- 

 ciall}", producing a broad triangular form, usually with blunt, rounded 

 tips. 



The abdomen is cut posteriorly by a median sinus which runs for- 

 ward l)etween the lobes very varying distances in the different species. 

 It may be broad, shallow, and well rounded {eatostoiiii), broadly triangu- 

 lar {maeidosus, lepldostei), narrowly triangular and cut deeply {stizos- 

 tethil^ niger)^ or so narrow as to be slit-like {versicolor). 



The anus is situated at the base of the sinus, and somewhere along 

 the sides or at the base are to be found the anal papilh^^. 



The position of these papilhe as subterminal, lateral, or basal adds 

 another secondary specitic character which is very useful in classifica- 

 tion. 



The appendages. — On the ventral surface of the carapace we find the 

 cephalic and the first thoracic paired appendages, and along the median 

 line the sting and the mouth apparatus. 



The two pairs of antennai are situated in front of the eyes and lie in 

 shallow troughs or depressions in the ventral surface of the carapace. 

 The first antennae have a broad flat basal portion which is two-jointed; 

 the proximal joint is much the smaller of the two, triangular in shape, 

 and is prolonged backward into a stout blunt chitinous spine. 



The distal joint is considerably elongated at right angles to the prox- 

 imal one (and hence at right angles to the central axis), and it termi- 

 nates in a strong sickle-shaped hook which is curved over ventrally. 

 There is usually a similar but much smaller hook upon the anterior 

 margin near the base of the joint, and in most species a stout spine, 

 occasionally two of them {versieolor), upon the posterior margin. 



The terminal portion of the antenna which is three-jointed is so 

 diminished in size and relatively so insignificant as to be easiiy over- 

 looked. Furthermore, it is fastened to the distal joint alongside the 



