Prof. Allman on Chelura terebrans. 365 



whose structure is peculiar. These setse are bulbous at their 

 origin, then present for some distance a cylindrical stem, but 

 soon become suddenly attenuated, and from this point to the ex- 

 tremity are beautifully pectinated on one side (fig. 13). They 

 are for the most part arranged in linear groups of from three to 

 five. The setse which immediately surround the base of the last 

 joint are of the ordinary structure. 



The second pair of feet (fig. 14) resembles the first in all es- 

 sential points ; it is however without the pectinated setae, and its 

 antepenultimate joint, which is narrower than that of the first, 

 supports near its distal extremity a row of small curved spines 

 constituting a kind of comb-like organ. The five pairs of feet 

 which succeed to these (fig. 1 & fig. 14^) are all nearly similar 

 to each other, and consist of six joints, of which the terminal 

 one is in the form of a short claw, but does not by opposition to 

 a produced portion of the penultimate joint constitute, as in the 

 first two pairs of feet, a didactyle prehensile organ. The third 

 and fourth pairs resemble the first two in being directed for- 

 wards, the last three pairs point backwards. In none of the 

 thoracic legs does the first joint present the shield-like form so 

 frequent among the Gammarida. 



The thoracic legs are copiously beset with long hairs, for the 

 most part plumose ; a flattened membranous branchial vesicle is 

 borne at the base of each (fig. 14), and in the females there is also 

 found in the same place another appendage in the form of an 

 oblong membrane (fig. 14) set round its edges with blunt spines, 

 and probably destined for the protection of the eggs. 



In the abdomen five distinct segments may be demonstrated. 

 The first two resemble those of the thorax, but are narrower, 

 and not furnished with distinct epimerse. The third is large, 

 also without distinct epimerse, and is produced posteriorly by a 

 long curved spine which points towards the tail, and has a small 

 tubercle or rudimental spine at each side. These three segments 

 carry each a pair of biramous natatory feet, each foot (fig. 15) 

 consisting of a basal lamina supporting two long, flat, transversely 

 striated rami which are copiously furnished on the edges with 

 long, beautifully plumose setse (fig. 16). These natatory feet are 

 kept in a constant state of rapid vibration backwards and forwards 

 both while the little animal is swimming and while it is holding 

 on by its true feet, and they would seem, by the extensive cur- 

 rents which they produce in the surrounding fluid, to be subser- 

 vient to respiration as well as to locomotion. 



To the three segments just described there succeeds a very 

 large one (figs. 1, 17), equal in length to about twice that of the 

 preceding three together. It is of a somewhat oblong figure 

 compressed horizontally, with a slightly prominent keel at each 



