CRUSTACEANS. 607 



external, of firmer structure and commonly beset with hairs at the, 

 margin, protects as a gill-cover the innermost soft and sacciform 

 slate, the proper gill. The normal number of gills is five pairs, but 

 in the land Oniscides and Asellus there are only three pairs deve- 

 .oped. In Asellus and many marine Oniscides, the gill-covers 

 themselves contribute to respiration 1 . In most crustaceans indeed 

 ;hese two chief forms of plates or filaments become modified through 

 greater development, and each gill consists not of a single plate or 

 of a single thread, but of a large number of plates or threads. Thus 

 .n I/imulus, on the upper surface of the five last abdominal feet, 

 which have assumed the shape of flat semicircular discs, there are 

 bund five pairs of gills, each of a hundred plates or more, whilst 

 ;he first pair of abdominal feet not bearing any gills at their base, 

 t the external sexual organs, covers all the succeeding feet on the 

 under-surface, after the manner of a gill-cover. In Squilla there are 

 ive pairs of gills, in the form of numerous filaments placed pecti- 

 nately on a pedicle, which are attached to the base of the fin-like 

 posterior feet. In the ten-footed short-tailed crustaceans there are 

 commonly seven gills on each side, of which that in the middle is 

 the longest. They have a pyramidal form and are divided longi- 

 tudinally by a middle septum from the base to the apex ; on this 

 septum numerous plates are set at right angles which make up the 

 Dyramidal body of the gills, and consist of folds of the double 

 membrane of which the septum is composed; these plates thus 

 form sacs which may be inflated through the septum. In some long- 

 tailed decapods also the gills are leaf-shaped but in most they con- 

 sist of a multitude of fine cylindrical filaments which are attached 

 instead of plates to each side of the axis of the gill. The gills are 

 more numerous, sometimes even twenty-one on each side. In all the 

 decapod crustaceans the gills are situated under the lateral parts of 

 the shell (carapace) in a proper cavity on each side, and are attached 

 to the basal piece of the five pairs of feet, or at the same time to the 

 tiindmost foot-jaws also. The water penetrates to the respiratory 

 cavity by an opening on each side at the inferior margin ; in the 

 short-tailed this opening is situated in front of the basal piece of 



1 TREVIRANUS Verm. Schr. I. s. 6062, Tab. ix. figs. 50 52, s. 73 75, Tab. 

 xu. figs. 63 65. Compare also DUVERNOY et LEREBOULLET Essai d'une Monographic 

 des organes de la respiration de Pordre des Crtistace's isopodcs. Ann, des Sc. not. IQ Srie, 

 Tom. xv. Zoolog. pp. 177 240, PI. vi. 



