108 ISOPODA. 



Mr. Dana insists that the Isopocls have but a single cord 

 between the ganglia, whilst it is doubled in the Amphi- 

 pods, founding thereon an argument of inferiority in the 

 double-corded Amphipods, the single cord only appearing 

 in the higher Crustacea, whilst its existence in a double 

 state indicates a diminution in the nervous influence of 

 the creatures. He has omitted to state upon what 

 species he has founded his observations ; but Brandt 

 (Med. ZooL, vol. ii. p. 76, PL XV., fig. 28) decidedly 

 figures the nervous system of the Oniscidas as consisting 

 of a double cord ; and the same fact has also still more 

 recently been clearly demonstrated by Franz Leydig in 

 his excellent " Tafeln z. Vergl. Anat.," 1864, PI. VL, f. 7, 

 representing the nervous system of Porcellio scaber. 



According to Edwards and Audouin, there is a gan- 

 glion to each segment of the pleon. 



The male external organs of generation consist of a 

 pair of linear or filiform appendages or hooks near the 

 base of the hind legs, or between the base of the anterior 

 pair of appendages of the pleon, except in Anceus, which 

 Mr. Hesse has recently demonstrated to be the male of 

 Praniza, in which the organ is a single median intromit- 

 tant process. 



The young when hatched differ from the perfect ani- 

 mals in possessing a narrower form, and with the limbs 

 not fully developed, and in some genera with fewer seg- 

 ments to the pereion at the period that the larva quits 

 the egg. During the period of embryonic existence, 

 according to the observation of Dr. Fritz Miiller, the 

 animal within the ovum is rolled outwards except in the 

 aberrant Isopoda, which in this resemble the Amphipoda 

 by being rolled inwards. 



The classification of the Isopoda, with reference to the 

 aflinities of the different animals amongst themselves and 



