ISOPODA. 107 



into branchiae variously arranged, as above described ; but 

 in the terrestrial species of the Oniscidae a remarkable 

 modification exists in the structure of these same appen- 

 dages, to fit them for performing the ofiice 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 antennae 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 



