108 ISOPODA. 
Mr. Dana insists that the Isopods 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 Oniscide 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, Pl. VI., 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 Isopopa, with reference to the 
affinities of the different animals amongst themselves and 
