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NERVOUS SYSTEM. (Comparative Anatomy.) 
lusca, the latter in the Articulata. In the 
Mollusca, what is it that constitutes an increase 
in developement of the primary nervous ring, 
the characteristic form of the nervous system 
of that class? 
1. The greater volume of a central medul- 
lary mass, and its situation on the dorsal aspect 
of the animal. 
2. A small number of ganglions in the pri- 
mary nervous ring, proportioned to the deve- 
lopement of the muscular system, one predo- 
minating in size over the rest, especially if that 
‘one be situated on the dorsal aspect of the 
animal. These ganglia are disposed unsym- 
metrically throughout the body, whence Pro- 
fessor Owen has designated these animals, in 
ete, to their nervous system, Heterogan- 
al. 
- Mottusca.—1. Tunicata—Many of the 
Tunicata, the lowest of them, approach in cha- 
racter the Zoophytes ; for no particular medul- 
_ lary mass constituting a nervous system is dis- 
coverable in the soft texture of their bodies, 
_ except in but few of the genera, principally in 
the forms of Ascidie. In the Ascidia mam- 
“millata (fig.336), Cuvier describes and figures 
the nervous system* as consisting of a single 
oblong ganglion, situated near the anus of the 
animal, and between that and the branchial 
Fig. 336. 
Wa 
Ascidia Mammillata, (after Cuvier, ) shewing the 
single gangliun between the branchial and anal 
ort . 
* Anatomie des Mollusques. 
603 
orifice; from this ganglion branches ae given 
off, some of which, passing to the esophagus, 
encompass it in the form of a ring. 
[ Mr. Garner, in his valuable paper on the 
nervous system of molluscous animals, de- 
scribes the nervous system of Phallusia intes- 
tinalis. The single yellowish ganglion lies 
upon the muscular coat between the two ori- 
fices. One set of filaments coming from it 
surrounds the branchial orifice, and gives nerves 
to its tentacula, appearing to meet on the oppo- 
site side, forming a nerve which seems to run 
along the edge of the elongated branchial fold. 
The other set supply the muscular tunic and 
go towards the mouth. In Cynthia and those 
tunicata that have thick muscular tunics, the 
ganglion is not visible external to the muscular 
sac, it being situated in its interior. 
As the actions of these animals are ex- 
tremely simple, so is their nervous system: by 
the branchial orifice water is drawn in to supply 
the branchie and to convey nutrient matters 
to the mouth. It is propelled by the action of 
numerous cilia which cover the surfaces with 
which it comes in contact. Through the anal 
orifice are expelled both the current which sup- 
plied the respiratory surface and that which 
passes through the digestive canal. Each ori- 
fice is provided with a sphincter muscle, which 
may oppose the entrance of various matters at 
one orifice, or resist their exit at the other. These 
muscles receive filaments from the ganglion. 
The animal is surrounded by a muscular sac, 
which by its contraction can compress and 
empty its general cavity. This, too, receives 
some nervous filaments. The solitary ganglion 
of this ascidian seems to regulate the actions 
of its orifices of ingestion and egestion, and of 
its enveloping sac on which depends the slight 
locomotive power of the free species. We are 
not prepared to deny to even this simple being 
that prevailing attribute of animals, a will, and 
therefore it may be assumed that its actions are 
partly volitional and partly reflex (mental and 
physical)—while some are, no doubt, due 
simply to the inherent irritability of its muscu- 
lar tunic. 
2. Conchifera—In this order the nervous 
system is precisely adapted to the functions these 
animals have to perform. These are ingestion 
of the food, respiration, and locomotion. Their 
nervous centres or ganglia are, consequently, 
placed in immediate relation to the organs 
destined to those functions; but as one pair 
communicate with the others, it may be pre- 
sumed to exercise an influence over them, and 
to be the principal centre, the analogue of the 
brain in the higher animals, the focus of sen- 
sation and volition. 
The esophageal or labial ganglia are for this 
reason the most important. They are two in 
number; they are situated more or less near 
the mouth, and are united bya transverse branch 
which arches over it. From these ganglia 
nerves are given off to the mouth, and to the 
tentacles, and to the anterior parts of the vis- 
cera. Each ganglion has a branch of commu- 
nication to the pedal ganglion and also to the 
branchial ganglion. 
