MOLLUSCA. 
‘and complex, and in the Cephalopods is evi- 
dently provided with gustatory papille. The 
sense of touch is in this as in other groups of 
animals the most generally possessed and the 
most widely diffused over the body. The mar- 
inal fringe of tentacles on the mantle of most 
valves: the branched processes of the skin 
in certain Pteropods and Gastropods ; the ce- 
halic tentacles or ‘ horns,’ as they are termed, 
in the Snail and allied Mollusks; the rich and 
varied apparatus of cephalic and labial ten- 
tacles—nearly one hundred in number—in the 
Nautilus; and the extremities of the acetabuli- 
ferous arms of the higher Cephalopods, may 
all be regarded as organs of delicate sensa- 
tion: the same opinion may reasonably be 
entertained of the highly vascular surface of 
the ventral locomotive disc or ‘ foot’ in the 
Slug, Snail, and other Gastropods. 
The voluntary muscular fibre of the Mollus- 
cous animals is distinguished from that of the 
Articulate and Vertebrate animals by the ab- 
‘sence of the transverse strie, and in most of 
the Acephalous Mollusks it is antagonized by a 
merely elastic gelatinous fibre. In the Tuni- 
caries the flexible outer coat obeys and opposes 
the change of form which the inner muscular 
envelope occasions. In the Bivalves, whose 
shell affords firm levers of attachment to the 
muscles, the antagonizing elastic force is im- 
planted at the hinge of the shell; and some of 
the species ( Mollusca subsilentia of Poli) can, 
by virtue of this mechanism of solid lever with 
its attached muscular and elastic fibre, execute 
short leaps. 
_ The Encephalous Mollusks with a cerebral 
centre of nervous influence antagonize one 
series of muscles by the regulated action of 
another series, and are no longer dependent on 
mere elastic fibres for their movements. The 
musculo-cutaneous mantle is propeced in the 
form of fins in the Pteropods, Heteropods, and 
some Cephalopods ; or there is an accumula- 
tion of longitudinal fibres, intersected by ob- 
ique or transverse ones on the ventral surface 
of the mantle, producing the thick contractile 
disc, which is termed the foot. This some- 
times extends the whole length of the body, as 
in the Gastropods, sometimes is developed onl 
from the region of the neck, as in the Trache- 
lipods. The attachment of the body to the shell 
in these Mollusks, the presence of, and power 
of retracting and elongating, a siphon or breath- 
ing tube, the movements of the head and its 
appendages, especially when these are deve- 
loped into instruments of progressive motion 
and prehension, as in the Cephalopods, are the 
Shic! conditions of the progressive advance- 
ment and complication of the muscular system 
in the Molluscous sub-kingdom. 
_ The heterogangliate type of the nervous sys- 
tem, with the correspondingly low condition of 
the muscular and other organs of relation, 
which, commencing in the Ciliobrachiate Po- 
lypes, is established in the Mollusks, is on the 
whole very inferior to the conditions and powers 
of the sensitive and motive systems in the Arti- 
culates: but, on the other hand, “ the organs 
of growth and reproduction become more 
365 
evolved ; and in the Mollusca we are presented 
with a perfecting of the internal organs, which 
is to prepare for and to be more fully deve- 
loped in the higher animals.”’* 
The alimentary canal is provided with a se- 
parate mouth and vent: the stomach may be 
distinguished from the csophagus and intes- 
tine, and a liver is present in all the Mollusks, 
and is remarkable in most for its large size and 
complicated lobular form. The bile is secreted 
from arterial blood. The mouth in the Ace- 
phalous Mollusks is generally concealed in the 
interior of the pallial cavity, but is always si- 
tuated at the anterior part of the body, or that 
which is opposite to the excrementory and re- 
Spiratory tubes or orifices. There are neither 
jaws nor salivary glands in this division. Amon 
the Encephala, however, in which the mouth 
is armed with horny plates representing in diffe- 
rent species the knife, the saw, the rasp, or the 
scissors, the salivary system attains an extraordi- 
nary degree of development, especially in the 
Cephalopods, in which the mandibles resemble 
those of the Raptorial bird, and sometimes have 
their margins armed with a calcareous dentated 
plate. In the Cephalopods the pancreas makes 
its first appearance. The special forms of the 
progressive complications of the digestive sys- 
tem in the present division of animals will be 
found amply illustated in the articles Tunt- 
cata, Concuirera, Preropopa, Gasrro- 
popa, CEPHALOPODA. 
The circulating system, which, as has been 
stated, is complete and double in all the 
Mollusks, is provided in all the species with 
a systemic heart, and in the highest organized 
species with a distinct heart for the lesser or 
branchial circulation, The systemic heart first 
appears in the sessile Tunicaries as a vasiform 
undivided ventricle, which, however, is in- 
closed in a distinct pericardium. It is concen- 
trated into a more compact muscular organ in 
the Conchifers, and divided into a venous and 
arterial chamber; but the auricle, and some- 
times also the ventricle, as in Arca, is sub- 
divided and segregated, according to the law 
of self-repetition, which is exemplified in all 
the systems of organs at their first appearance 
in the animal kingdom. The heart of the 
Gastropods exhibits the higher type of a single 
auricle and ventricle, both placed at the termi- 
nation of the lesser and the commencement of 
the greater circulation. In the highest Cepha- 
lopods the course of the blood is accelerated 
in both circulations by a muscular ventricle, 
but the superadded analogue of the pulmonic 
heart here, likewise, at its first appearance 
illustrates, by its separation into two distinct 
ventricles, the law above alluded to in reference 
to the systemic heart of the lower Mollusks. 
The respiratory organ is distinctly developed 
in all Mollusks, and is subject to the greatest 
variety of forms in this division of the animal 
kingdom; yet, with the exception of the small 
sessile order of Ascidians at the lowest extreme, 
* See the eloquent and ‘philosophic ‘ Recapitu- 
lary Lecture’ of Prof, Green, Vital Dynamics, 8vo. 
1840. 
