364 
or bluish, and it appears to contain a smaller 
proportionate quantity of fibrine than that of 
the Vertebrata. The veins probably fulfil the 
functions of absorbent vessels. Their muscles 
are attached to various points of their skin, 
forming more or less dense and complex tis- 
sues. Their motions consist of various con- 
tractions, which produce inflections and _pro- 
longations of their different parts, or a relax- 
ation of the same, by means of which they 
creep, swim, and seize upon various objects, 
just as the form of these parts may permit, 
ut as the limbs are not supported by articu- 
lated and solid levers they cannot advance 
rapidly. Nearly all Mollusks have the body 
enveloped by a duplicature of soft and usually 
muscular integument, which bears more or less 
resemblance to a mantle: it is sometimes nar- 
rowed into a simple disc, sometimes prolonged 
into one or more tubes, or is extended and 
divided in the form of fins. 
The Naked Mollusks are those in which the 
mantle is simply membranous or fleshy. In 
most of the species one or more plates, of a 
substance more or less hard, are a in 
its substance, usually in successive layers, 
which increase in extent as well as in thick- 
ness as they are successively formed. When 
this substance remains concealed in the thick- 
ness of the mantle, it is still customary to style 
the animals ‘ Naked Mollusks.’ Most com- 
monly, however, it becomes so much deve- 
loped that the contracted animal finds shelter 
beneath or within it, and it is then termed a 
shell, and the animal is called a Testaceous 
Mollusk. 
The mode of generation is too varied in the 
Mo!lusks to afford any common character to 
this sub-kingdom : but, being for the most part 
sluggish and feeble animals, with low and little 
varied instincts, they are preserved chiefly by 
their fecundity and vital tenacity. 
The nervous system of the Mollusks may 
consist of one or more ganglionic masses ; but 
these are scattered, often irregularly or unsym- 
metrically, through the body, and the inter- 
communicating chords never form a symme- 
trical knotted pair along the middle line of the 
ventral surface of the body; whence the term 
* Heterogangliata,’ expressive of the charac- 
teristic condition of the nervous system, with 
which the unshapely and often unsymmetrical 
figure of the whole body corresponds. 
The number of the ganglia follows closely 
the progressive development of the muscular 
system : the first is that which is found between 
the anal and respiratory tubes of the Ascidiz, 
and which regulates the elongator and sphincter 
muscles of these tubes. The development of 
a muscular heart and of a bivalve shell with 
its adductor muscle is accompanied with the 
appearance of a second ganglion or centre of 
the nerves which supply that muscle: a second 
adductor muscle, with the superaddition of a 
muscular foot and its retractors, produces an 
additional ganglion or ganglions, and the com- 
plication of the nervous system is further aug- 
mented when the breathing and anal siphons 
MOLLUSCA. 
are unusually prolonged, and provided with — 
strongly developed annular contractile muscles 
and with proportionately powerful retractors. 
The progressive development of the nervous 
system may be traced thus far in the tunicated 
and bivalve Mollusca without its reaching the 
stage which is marked by the ap 
z 
+ 
ove. 
distinct supra-cesophageal ganglion or brain. — 
the species have, in fact, no distinct head, and 
are termed Acephalous Mollusks. The first 
appearance of this important part with its 
appendages, which are usually subservient to 
e organs of special sense, is associated with 
- accumulation of nervous matter, in the form 
of a transverse chord, with a lion or gang- 
lions, above the paRebeesal co of the cso- 
Phagus, forming the brain; whence these Mol- 
usks are termed ‘ Encephalous.’ 
The development of the brain proceeds 
directly as the organs of the senses increase in 
number and complication, and consequently 
pnt its maximum in the pearask : alo 
s with highly complex eyes and disti 
organs of Nidal’ pa in these the beta 
protected by a cartilaginous cranium, pier 
the first representative of the true interna 
skeleton which is met with in the Invertebrate 
division of animals, and one of the main 
organic characters by which the higher Mol- 
lusks surpass Articulates in the ascent to the 
Vertebrate type. 
The nervous system of the Mollusks is re- 
markable for the peculiar and distinct colour 
of the ganglions in certain species, as the fresh- 
water Muscles and Unios; and for the contrast 
between the density of the cellular sheath of 
the nerves and the semifiluid pulp which it 
contains. This structure allows of an artificial 
injection of the nerves, which has led some 
anatomists to describe them as parts of an 
absorbent system. 
Although the bivalve Mollusks are headless 
and without brain, some of them have mani- 
fested signs of a perception of light, and in a 
few species simple ocelli have been detected on 
the verge of the mantle. we 
In the Encephalous Mollusks the eyes, 
when present, are small, never exceed two in 
number, and are usually supported on flexible 
uncles or tentacula; but in the Dibran- 
chiate Ay eatin: they are large, always ses- 
sile, and highly complicated. ae 
The organ of hearing is liar to the Ce- 
ras oy in the present division of the animal 
"theo special and cine 
e organ of smell, as a i cir- 
cumscribed part, has likewise been recognized 
only in the highest class: but Cuvier observes, 
that the skin of the Mollusks so clearly re- 
sembles in its softness and lubricity a pituitary 
membrane, that they probably may recognize 
odours at every point of their external surface. 
Professor De Blainville conjectures that in 
Gastropods the soft extremities of the first 
of cephalic tentacles may be the seat of 
organ of smell. An organ of taste has 
course been recognized only in the Encephalous - 
Mollusks, in many of which the tongue is large 
pair: 
of 
