608 
of an irregular white line, the structure of 
which, when viewed under a lens, appeared 
to consist of a number of closely approxi- 
mated nervous threads. In a larger species the 
nervous cord had a more jointed, knotted ap- 
pearance, and its structure was more homoge- 
neous; the ganglia were, however, but very im- 
oy, developed, for at each segment of the 
y of the worm, the nervous cord only offered 
an enlargement or swelling of rather an elongated 
form (fig. 344, d): the first or most anterior 
Fig. 344. 
Hoy 
Upper fourth of the nervous cord of Lumbricus 
terrestris (earthworm ). 
a, supra-csophageal or 
e, infra-cesophageal ganglion. 0, b, lateral ner- 
vous cords, forming the oral primary nervous 
ring. d,d,d,d, enlargements of the ventral cord, 
or imperfect ganglia, developed on the ventral 
surfaces of the series of primary nervous rings. 
cerebral ganglion. 
of these (the infra-cesophageal ganglion) was 
the largest (c). These caingeanents were more 
closely Ad aig weary towards the anterior part 
of the body; from each of them were given off 
two pair of nerves, one passing to the right, 
the other to the ieft, to supply the integuments. 
From the more attenuated intervening portions 
of the cord, a single pair of filaments was given 
off; these filaments we at once recognize as the 
commissures of the nervous ring of the Radiata 
and Mollusca; they pass round the body and 
approach each other on the dorsal surface, but 
do not unite: in this way is an open nervous 
ring formed. The infra-cesophageal ganglion 
before mentioned diverged at its anterior part, 
sent tee two lateral nervous cords (6), 
which developed a large bilobate cerebral or 
supra-cesophageal ganglion (a) of a transversely 
elongated oval form, thus forming a distinct 
nervous ring, embracing the esophagus. From 
the angles formed by the divergence of the 
infra-cesophageal ganglia, a nerve of rather firm 
texture was given off; another filament had its 
origin from the lateral and ascending portions 
of the nervous collar; and from ae eral 
ganglion two pairs of nerves passed forwards to 
supply the saits about the head. In the 
Aphrodita, a marine Annelide, the nervous 
NERVOUS SYSTEM. (Comparative Anatomy.) ¥ 
cord had a more flattened appearance: it was — 
about equal in width throughout its whole 
length, and presented similar enlargements in — 
its course, from which nerves were given of 
as in the Lumbricus; no nerve, however, arose 
from the intervening spaces. In the 
the abdominal nervous cord was surroundet 
by several delicate vessels, and at the anterior 
os of the commissures was distinctly seen to” 
composed of two separated columns: the 
ganglia were very distinct, of a round form, 
and twenty-four in number,—the last four o1 
five were more closely approximated; from 
each were given off two diverging pair of ate 
filaments, which passed to supply the muscles 
and viscera. The cerebral ganglion is bilobed, 
and sends off ten distinct optic nerves besides 
many smaller filaments to the integument and 
other of the head; each optic nerve T 
minates by expanding upon the base of a black 
eye-speck or ocellus.* 
According to Brandt, a simple nervous fi 
ment is continued from the cesophageal ganglia 
along the dorsal aspect of the alimentary 
“ This,” says Professor Owen, “is an inte- 
resting structure, since it offers the first trace 
of a distinct system of nerves, usually calle 
the stomato-gastric in Entomology, and to 
which our great sympathetic and nervus vagus 
seem answerable. 
These four classes comprise the whole ¢ 
the helminthoid Articulata; and on reviewins 
the statements just made with regard to their 
nervous system, we observe that the lowest 
them bear a great analogy to the Acrita 
some of the Tunicata, in having no distinct 
nervous system discoverable; that where such 
does exist in the higher orders of them, it bears 
more or less the character before noticed as the 
type of the Articulata, which was, it will be 
recollected, a series of primary nervous rings 
connected by hegre nN in the Ascari 
the ganglia are so imperfectly patie #~. nd 
consequently their conantaahiiae that hol 
has the appearance of only a single nervous 
filament. As we proceed through the Rotifer 
and Cirrhopoda, we observe the rathe 
more perfectly developed, but undetermined 
number. In the highest of the Annelides, 
the Leech, we find distinct ganglia develope 
and arranged in a determinate numerical ser 
Again, the cerebral ganglion, from bh 
first little more than a simple enlargement: 
the superior surface of the anterior prim 
nervous ring, has acquired, in the 
form of a distinct ganglion. 
We next pass to the examination of the | 
tomoid Articulata, in which we shall find 
cerebral ganglion becoming more and 1 
developed, and the ventral ganglia more : 
termined as to number, and more concentr 
as to situation; their longitudinal commissui 
have been supposed to two disti 
nervous tracts, composed of motor and s 
sitive columns, giving origin to nerves hav 
sentient and motiferous properties. . 
1. Crustacea.—In the lowest of the 
Crustacea, the nervous system preser 
* Owen’s Lectures, p. 141, 
ey | 
