INTRODUCTION. xxxvii 



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 Muller is quite positive in the assertion that no 

 corpuscles of the circulating fluid pass into the caudal 

 appendages, which are the seat of the branchiae in the 

 normal Isopoda. 



The terrestrial Isopoda 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 branchiae, 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 trachea! 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 

 branchiae. This view is supported by M. Milne Edwards, 

 as may be seen by the reference to the " Atlas du Regne 

 Animal," (Pl.lxx.fig.l. m.), and "Leyons sur la Physiologie 

 et PAnatomie comparee," t. ii. p. 141. Our ow r n opinion 

 relative to these organs on the branchial operculum is 

 that they are glands for the secretion of a fluid that 



