INTRODUCTION. XXXVil 
This appendage appears not to be constant in all species, 
nor in all specimens of the same species. If, therefore, it 
be the homologue of a branchial sac, it can only be an 
organ of repetition. 
Fritz Miller is quite positive in the assertion that no 
corpuscles of the circulating fluid pass into the caudal 
appendages, which are the seat of the branchie in the 
normal Jsopoda. 
The terrestrial Jsopoda have the respiratory organs some- 
what modified from those of the aquatic species. These 
have been described and figured by MM. Duvernoy, Sa- 
vigny, Lereboullet, and Professor Wagner. M. Savigny, 
however, was the first to show that in the genus Tylos 
the system of respiration was carried on by two separate 
means; the one by branchie, as in aquatic Crustacea, 
the other by the spiracular air-tubes. This has been 
recently confirmed by Professor Wagner, who shows the 
relation of the opercular valves to the respiratory system, 
and contends that, besides their power of protecting the 
branchial plates from injury, and precluding the too rapid 
escape of moisture, they fulfil, by means of a plexus of 
minute vessels, situated at the base of the operculum, a 
pulmonary function. This organ, which he figures, has, 
he says, a kind of tracheal division into numerous rami- 
fications. Seen by transmitted light it is opaque, but 
viewed under a direct light it is silvery white; and he 
contends that it is a pulmonary or tracheal chamber, 
which serves as a supplementary organ to the true 
branchiz. This view is supported by M. Milne Edwards, 
as may be seen by the reference to the “ Atlas du Regne 
Animal,” (Pl. 1xx. fig.1.m.), and “ Legons sur la Physiologie 
et PAnatomie comparée,” t. ii. p. 141. Our own opinion 
relative to these organs on the branchial operculum is 
that they are glands for the secretion of a fluid that 
