NERVOUS SYSTEM. (Comparative Anatomy.) 
may be considered as a repetition of the other; 
of these, the most anterior acquires the greatest 
developement, and is called the head. So in 
examining their nervous system, we shall find 
that a primary nervous ring (formed of a gan- 
glion and two semi-circular radiating nerves) is 
contained in each segment. This ring, no 
longer closed, as in the preceding classes, but 
open, varies in degree of developement, accord- 
ing as the segment which encloses it is in a 
high or low degree of developement: thus, in 
the cephalic segment, or head, we shall always 
find developed a cerebral or supra-csophageal 
_ ganglion; and, inasmuch as when a true ner- 
yous system was first formed—was first sepa- 
rated from the punctiform homogeneous mass 
of the gelatinous acrita—commissures were 
found uniting the primary masses of medullary 
_ Substance (as we saw in the Asterias), so ought 
| 
, 
we to find, in the Articulata, commissures 
uniting the primary nervous rings ; which latter 
are now become longitudinal, and the com- 
missures of the nervous ring itself are now 
become radiating nerves. We ought also to 
find that these commissures depend, in degree 
_ of developement and in situation, on the same 
_ characters of the primary nervous ring, and 
consequently on the ganglia thereon developed. 
We may next ask, what will mark the high 
or low degree of organization of a nervous 
_ System composed of several primary nervous 
_ rings? The researches of philosophical anatomy 
"inform us, that, first, a low degree will be 
characterized by an undetermined number of 
__ those rings—by a nearly equal developement of 
_ the whole, and by the central mass of nervous 
Matter accumulated on them being situated on 
the ventral surface of the animal. Secondly, a 
higher degree of organization exists when the 
primary nervous rings are repeated in a deter- 
Minate manner—when some of them predomi- 
nate in developement over the others, and their 
central medullary masses, or ganglions, are 
Situated on the dorsal aspect of the animal. 
Again, as regards the uniting commissures, 
these will, of course, depend, in degree of 
developement, on the organization of the gan- 
glions united by them; and the more perfect 
and the more intimate is the connexion esta- 
blished by these commissures, the more highly 
ees is the nervous system. 
n the Articulata about to be described, we 
shall always find the most anterior nervous 
ring developing a ganglion on its superior sur- 
true cerebral ganglion. We shall find 
this nervous ring repeated in the other segments 
of the body, but in a much more imperfect 
manner, for ganglions are developed only on 
the ventral surface of the animal; and from 
this latter circumstance, they, as well as their 
commissures, cannot be highly developed. 
1. Entozoa.—In the lower forms of Entozoa, 
as in the tenia and cysticercus, no nervous 
System is discoverable. These animals consist 
of a gelatinous, more or less homogeneous 
mass, in which no distinct nervous system 
exists. In the Distoma hepaticum, the nervous 
System consists, according to Bojanus,* of a 
* Isis, 1821, vol. i. p. 168. 
607 
nervous collar or ring, with two latex 1 gan- 
glions entwining the ceesophagus, and twO nerves 
which are distributed on the posterior part of 
the body. In the Ascaris lumbricoides, the 
nervous system consists of a thin double fila- 
ment, without ganglia, situated in the median 
line of the abdomen, which separates to enclose 
the opening of the vulva, and to encompass 
the esophagus at the lower part of the mouth. 
In the Strongylus gigas, according to Otto,* 
the median nervous filament consists of very 
closely approximated ganglia, thus advancing 
a step higher in organization, and approaching 
to the character of the true articulated classes. 
2. Rotifera.—The Rotifera are minute mi- 
croscopic animals: in them Ehrenberg has 
discovered and described a rather complex ner- 
vous organization, sufficiently so to justify their 
being ranked thus high in the scale of animated 
beings.t In the Hydatina senta, according to 
this anatomist, the nervous system consists of 
two closely approximated filaments running 
along the abdomen, and giving off lateral 
branches in their course forwards: arrived at 
the anterior part of the body, these nerves form 
a large ganglion, and then ascend to embrace 
the esophagus in the form of a ring, on which 
minute ganglia are developed, giving off nu- 
merous filaments to the surrounding parts. 
There are four of these lateral ganglia, besides 
the large supra-cesophageal ganglion. 
3. Cirrhopoda.—In the Cirrhopoda, the 
abdominal nervous cords have regular ganglia 
developed on them, and there is a nervous 
collar round the esophagus, as in the preceding 
classes. Cuvier observes, { that in a species of 
Lepas he found two nervous cords situated on 
the ventral surface of the body, with five double 
ganglia developed on them, from which were 
given off lateral filaments to supply the curled 
feet. Anteriorly, and at the lower part of the 
mouth, these cords separated more widely, to 
encircle the esophagus, above which they de- 
veloped a quadrilobate ganglion, from which 
were given off four nerves to the viscera and 
muscles. 
4. Annelida—The nervous system of the 
Annelida consists of a varied number of ganglia, 
united by double longitudinal commissures, 
running along the ventral surface of the body, 
from which lateral filaments are given off. 
There is also a supra-cesophageal ganglion, 
which, being connected by lateral nervous cords 
with the first pair of infra-cesophageal ganglia, 
form a ring or collar, surrounding the cesopha- 
gus: this we at once recognize as the most 
anterior of the column of primary nervous 
rings, with the ganglion developed on its supe- 
rior surface. I have examined the nervous 
system in the genera Lumbricus, Aphrodita, 
and Hirudo; the general plan was the same in 
all. In the Lumbricus terrestris, or common 
earth-worm, a nervous cord passed along the 
whole ventral surface of the animal, and pre- 
sented, in a small species, the appearance 
* Berliner Magazin, 1814, p. 178. 
+ Organisation Systematik der Infusions-Thier- 
chen, Berlin, 1830. 
} Anat. des Mollusques. 
