ISOPODA. LO7 
into branchiz variously arranged, as above described; but 
in the terrestrial species of the Oniscide a remarkable 
modification exists in the structure of these same appen- 
dages, to fit them for performing the office of respiration 
to animals which would be destroyed if immersed in the 
fluid from which the aquatic species are enabled to ob- 
tain their necessary supplies of oxygen. Instead, there- 
fore, of acting as gills, the two or four anterior pairs of 
these organs are transformed into lungs, their interior 
being furnished with canals into which the atmospheric air 
penetrates by means of orifices answering to the spiracles 
of insects, and which, although generally very minute and 
numerous, are of large size and very distinct in the genus 
Tylos, Latr., inhabiting the shores of the Mediterranean, 
for the details of which we are indebted to Savigny’s 
beautiful figures in the great work on Egypt. It is, 
moreover, worthy of notice, with reference to the corre- 
lation of organs, that in these terrestrial species the in- 
ternal pair of antennz are so extremely minute as to be 
scarcely visible, consisting of a small three-jointed style, 
whilst the exterior pair are of large size. 
The nervous system of these animals approaches nearly 
to that of the true insects, consisting of a double thread 
united together at intervals by ganglions, the second and 
six following of which send off nerves to the seven pairs 
of legs; thus showing that the two anterior pairs, 
although homologous with the two outer pairs of mouth- 
organs of the Decapods, are really legs in the true sense 
of the word. The segments composing the tail receive 
the whole of their nerves from the terminal ganglion; a 
fact of much significance with reference to the real ho- 
mologues of the segmental development of these Crusta- 
ceans. This description is founded on the observations 
of M. Cuvier and our own dissections of Ligia; but 
