OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
245 
Myriapoda as hardly to affect the general form and appearance of the animal, although 
influencing every structure in that part of the body in which the change occurs. 
Structure. — The brain of the Myriapod is formed by the aggregation of separate 
ganglia* placed above the oesophagus. The first pair of ganglia are always the 
smallest, and give origin on their front to the nerves of the antennae, the anterior pro- 
longations of the nervous cord. The second pair, immediately behind them, con- 
stitute, as in insects, the organs of volition, and represent the brain of Vertebrata. 
They are in reality, as I have elsewhere shown, the analogues of the corpora rjuadri- 
geminadf^ and give off nerves at their sides to the organs of special sense, — the eyes. 
They are always more developed than the ganglia of the antennse, and continue to 
increase in importance as we ascend to the most perfectly developed insects. They are 
of large size, even when those of the antennse are almost absent, as in the larvse of 
lepidopterous insects, and even when the organs of vision are entirely wanting, as in 
the whole of an extensive family of Chilognatha, — the Polydesmidce. They are placed 
transversely above the oesophagus in the form of a crescent, the side of each lobe 
being a little in advance of the posterior. They are connected on their under and 
external surface by two cords of nervous matter, which are prolonged downwards, 
one on each side of the pharynx, and constitute the crura cerebri , — with the united 
ganglia of the maxillse and mandibles, which form the analogue of the medulla ob- 
longata^— the commencement of the abdominal cord. 
In the Iulidce (Plate XI. figs. 1 and 2) these cerebral lobes ( b ) are convex, both on 
their upper and posterior surfaces, and their original separation is marked by a 
median sulcus, which is more or less evident in different individuals. In some it is 
almost entirely obliterated, and the two lobes are more closely approximated, indi- 
cating, perhaps, the greater extent to which this important division of the nervous 
system has been developed in them than in others in which the union of the lobes is 
less perfect. Each lobe is connected externally with the optic ganglion ( c ), which is 
of an elongated, oval, and slightly conical form, from which nervous filaments radiate 
outwards and downwards in a triangular fasciculus to the cornea ( d ). The fibres are 
extended almost close to the cornea before they are clothed with dark pigment and 
form the retina, the chamber of each lens being scarcely longer than wide, so that the 
eyes are fitted only for examining near objects, a condition entirely in accordance with 
the habits of these animals. The ganglia of the antennae (a) in lulus are very small, 
and are situated near the junction of the cerebral lobes with the optic ganglia, and 
each gives off its nerve directly into the antennae. From the enlargement of each 
* Since this paper was delivered to the Royal Society I have found that, in the embryo of Necropklxophagus 
( Geophilus ) longicornis, Leach, at the moment of bursting its shell, the brain is composed of four double gang-ha, 
the centres of a corresponding number of segments, which are then becoming aggregated together to form the 
single moveable portion of the head in the perfect animal ; so that the brain of the Myriapod, and probably also 
of all the higher Articulata, is, in reality, composed of at least four pairs of ganglia. — G. N., July 14, 1S43. 
f Observations on the Anatomy, Habits, and Economy of Athalia centifolia. 8vo, 1838, p. 10. 
% Op. cit. 
2 K 2 
