a 
Affinities of Pteronarcys regalis. 439 
conclusion similar to that which we arrive at from the anatomy of the di- 
gestive organs. The number of segments to the body in Pteronarcys and 
Perla is the same, fourteen in each, but that of the ganglia of the nervous 
cord is different. In Pteronarcys the nervous system is composed of the 
brain and cord with twelve Subcesophageal ganglia. The first of these, the 
analogue of the medulla oblongata of Vertebrata, is situated, as in other in- 
sects, in the head, immediately below the brain, or cephalic ganglia, and sup- 
plies the organs of manducation, the mouth and pharynx. The second, third 
and fourth, of larger size, are in the three segments of the thorax, one in each, 
supplying the organs of locomotion, the legs and wings; and eight smaller 
ganglia, the first of which is in the metathorax, at a short distance behind the 
great ganglion of that segment, while the remaining seven are in the abdomen. 
But the nervous system in Perla consists of the brain and only ten ganglia in 
the cord. Of these the medulla oblongata and thoracic ganglia are in their 
usual situation, but the meso- and meta-thoracic are larger than in Ptero- 
narcys, more especially the latter, owing to the fifth, or first of the smaller 
ganglia of the cord, having united with the metathoracic in Perla, during the 
changes of the larva and pupa, as I have elsewhere shown * takes place also 
in the metamorphoses of the Lepidoptera, together with a shortening of the 
cord in one or more of the basal segments of the abdomen. Owing also toa 
Similar cause, the shortening of the interganglionic portion of cord, the ana- 
logue of the sixth ganglion in Pteronarcys occupies the position of the fifth 
of that genus in Perla, the sulcus of the metafurca (w); while the seventh 
of Pieronarcys is situated in Perla at the anterior of the basal segment of the 
abdomen, and is separated from the preceding ganglion only by a very wei 
portion of cord. Similar alteration in position, with coalescence of ‘ganglia, 
seems to have taken place at the termination of the cord in Perla, in which 
the eleventh and twelfth ganglia of Pteronarcys seem to have become Malen. 
This may account for the remarkable difference in the number of gmigtia in 
these two genera. Perla is thus as much in advance of Mionancys in the 
general structure of its nervous system, as in that of its dupsitive and — 
ratory organs. Approaching as these two genera do in their entire organiza- 
tion to the Orthoptera, they seem to represent some of the lower forms of the 
, * Philosophical Transactions, 1834, part ii. 
‘VOL. XX. : 3M 
