CRUSTACEANS. 607 
external, of firmer structure and commonly beset with hairs at the 
margin, protects as a gill-cover the innermost soft and sacciform 
plate, the proper gill. The normal number of gills is five pairs, but 
in the land Oniscides and Asellus there are only three pairs deve- 
loped. In Asellus and many marine Oniscides, the gill-covers 
themselves contribute to respiration’. In most crustaceans indeed 
these 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 
in Limulus, on the upper surface of the five last abdominal feet, 
which have assumed the shape of flat semicircular discs, there are 
found five pairs of gills, each of a hundred plates or more, whilst 
the first pair of abdominal feet not bearing any gills at their base, 
but the external sexual organs, covers all the succeeding feet on the 
under-surface, after the manner of a gill-cover. In Sguilla there are 
five 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 
pyramidal 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 
hindmost 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. 1. s. 60—62, Tab. IX. figs. 50—52, s. 73—75, Tab. 
x11. figs. 63—65. Compare also DuvERNoY et LEREBOULLET Essai d’une Monographie 
des organes de la respiration de Vordre des Crustacés isopodes. Ann. des Sc. nat. 2e Série, 
Tom. xv. Zoolog. pp. 177—-240, Pl. vr. 
