602 
Cuvier has also been unable to make out any 
traces of a nervous system, and doubted the 
accuracy of the statement made by Spix. These 
animals are, however, extremely sensible to the 
touch, when expanded, and to the light when 
exposed to its influence. This would indicate 
some degree of nervous sensibility, but which 
we can conceive to be afforded by the nervous 
elements being distributed in their homoge- 
neous structure in a manner similar to the 
preceding classes. In the next class,” the 
Acalepha, which consists of gelatinous marine 
animals, Trembley,* Gede,+ Carus,} and other 
anatomists have failed to detect any distinct 
nervous filaments. Dr. Grant,§ however, de- 
scribes what he considers a nervous system in 
the Beroé pileus, and describes it as “ consist- 
ing of a double circular nervous filament, 
situated around the oral extremity of the body, 
which sends off minute filaments in each of 
the spaces between the eight longitudinal bands 
of cilie; these eight points, from which the 
longitudinal filaments come off, present minute 
ganglionic enlargements.” This statement has 
been recently called in question, and it is pro- 
bable that the nervous system in these animals 
is diffused throughout the gelatinous mass of 
which their bodies are composed. Dr. Milne 
Edwards describes and figures part of the 
nervous system in a larger species of Beroé 
( Lesueura vitrea ), as radiating from a single 
small ganglion which is closely connected with 
a coloured eye-speck, situated at the middle of 
the superior extremity of the body.|| 
Raprata.—In the next group of animals, 
the Radiata, nervous filaments are for the 
first time discoverable; and this being the 
case, it is important that we should notice 
what form and direction they assume: it is 
that of a ray and a central point, or a nerve 
and a ganglion; of these several are de- 
veloped; and as it is the very essence of a 
nervous system that it should consist of gan- 
glions united and not separated, threads of 
communication are developed, called commis- 
sures, and a ganglionic system is formed, the 
inferiority of which is expressed in the Echi- 
nodermata by the perfect equality of all the 
ganglions: these ganglions are also situated at 
an equal distance from each other, and are 
determined in their number and origin by the 
general organization of the animals: thus we 
shall find that in the Asterias, or star-fish, with 
five rays, there are five ganglions (with radia- 
ting nerves) sending off commissures, which, 
inasmuch as they are situated on a spherical 
surface, unite them in the form of a ring. 
(See Eco1nopERMATA, i's. 23, p. 44, vol. ii.) 
This ring we may call the primary nervous 
ring; it is that form which we shall hereafter 
recognize as the essential base of even the 
* Mémoires pour servir 4 l’histoire d’un genre 
de porypes d'eau douce, 1774. ees 
t oa zur Anatomie un siologie der 
Medusen, 1816. serenyd 
Anatomie Comparée, vol, i. 
Lectures on Comparative Anatomy. 
Ann. des Sc. Nat. n.s, t. xvi., and Owen’s 
Lectures, by White, p. 106. 
NERVOUS SYSTEM. (Comparative Anatomy.) 
+] 
j 
most varied forms of a nervous system. Tt is — 
only in the genus Asterias that a nervous ystem 
has been distinctly seen; and we are indebted — 
to Tiedemann for the first description of it, in” 
his Monograph of the Echinodermata.* In a 
small species of this genus, it consisted of 
a circular cord around the mouth, from 
which proceeded a filament along each ray, 
having, at its origin, a minute ionic en- 
largement ; the nervous ring u 
extreme edge of the central aperture in the 
careous frame-work of the body, and the fil 
ments rested on the inferior surface of the 
concealed by and at the base of the tubula 
feet and suckers. Two other filaments, muce 
shorter than those just described, according to 
Tiedemann,t are given off from each of the 
ganglionic enlargements, to be distributed te 
the stomach and other viscera. And Ehrenber 
affirms that the red points situated at th 
tremity of each ray are eyes, and receive nert 
connected with special ganglia. The stateme 
however, has not received confirmation fr 
any subsequent observer; but Mr. E. Fe 
describes a kind of protective apparatus 
taining to these points, consisting of a pe 
arrangement of the spines around them. [T 
existence of ganglia is questioned by man 
observers. Microscopic examination we 
decide the point. ] 
In the inus no nervous filaments fh 
hitherto been discovered; but in the gen 
Holothuria, Cuvier observes, “ that there a 
pears to be a very attenuated nervous 
around the cesophagus.”{ This Delle Chi: 
denies entirely.6 Dr. Grant, however, d 
scribes their nervous system to exist in 
form of a collar around the anterior part | 
the body, giving off longitudinal filaments. 
[{t is remarkable that, although the nervy 
system be very obscurely developed in thes 
animals, the action of the muscular integum 
is extremely powerful. The slightest irri 
of the surface is sufficient to occasion fo 
contraction of the integument to such a de 
that the thin membranes of the cloaca bec 
lacerated, and large portions of the inté 
and other viscera are forced from the 
aperture. “So common is this occurre 
says Professor Rymer Jones, “ that the 
anatomists were led to suppose that, | 
natural instinct, the ani . Ww S 
vomited their own bowels. It is, in 
extremely difficult to obtain speci 
of the Holothuride from the constant o 
rence of this accident.” 
We may next ask what are the chi ! 
of an increase in developement of the pri 
nervous ring just mentioned, as the ~ 
mental form of every nervous system. — 
are precisely these: either that it is it 
more highly developed, or that it is multi 
and repeated several times. This we shi 
illustrated in nature; the former in # 
* Anatomie der Rohrenholothurie, &c. 1 
t Loc. cit. “y 
¢ Régne Animal, vol. iv. 
§ Memoir sull’ asteria, &c, 1823. 
{| Lectures on Comparative Anatomy. 
