338 
MOLLUSCA. 
Mollusca. among the porcellaneous shells, it would separate the olives 
~~" from cones, the former being destitute of an operculum, 
while the latter possess one. These two genera, however, 
belong to a natural family, the animals of both genera hay- 
ing a respiratory tube upon the head, and the eyes placed 
on the sides of the tentacula, instead of being situated, as 
in the other gasteropoda, on the tips or at the base. This 
circumstance is calculated to convince us of the necessity 
of caution in the admission of characters. These may at 
first appear to be of extensive occurrence, and well adapted 
for the formation of families, but unless they exercise some 
visible influence on the animal, they can never be employ- 
ed with propriety in a natural system, however convenient 
they may be in an artificial arrangement. Operculated 
shells may be considered as generically different from those 
which are destitute of that organ, without any injury to the 
natural method. It would even, in many instances, be con- 
venient. 
Amongst univalve shells, considerable differences are ob- 
servable in the shape and position of the cavity of the shell. 
In some cases this cavity is simply conical, while in others 
it is conico-tubular, either revolving horizontally round a 
centre, or spirally twisted upon an axis or pillar. These 
circumstances furnish characters of great importance in an 
artificial system, as by means of them all testaceous bodies 
may be arranged into two tribes, the one possessing a pillar, 
round which the tube of the shell is twisted, while the other 
is destitute of any pillar. The former have been termed 
Stulidia, the latter Astulidia. As a natural character, 
however, these distinctions are of inferior importance, and, 
if employed would occasion a separation between the genera 
Planorbis and Lymnza, which are demonstrated by Cuvier 
to be nearly related. In the formation of genera, it may 
be employed with advantage, even in a natural system, aided 
by the structure of the pillar, and the direction of the whorl. 
The last character which we have to notice while speak- 
ing of the univalves, depends on the circumstance of the 
cavity being entire, or divided into chambers, being unilo- 
cular or multilocular. In the multilocular testacea there 
are a number of transverse plates, in some species perfor- 
ated, in others entire, which cross the cavity of the shell, 
and, in general, divide the external cavity, in which the 
animal resides, from the older and smaller ones, from which 
it has receded. In an artificial arrangement, such distinc- 
tions may be employed with advantage, even in the forma- 
tion of the primary divisions, but we entertain doubts as to 
the propriety of using them in a strictly natural method. 
We are in a great measure ignorant of the animals which 
inhabit the multilocular shells, yet as far as our knowledge 
goes, we are induced to regard the distinction as merely 
conventional, and as unconnected with any peculiar order 
of organization. Such a division may be useful in the pre- 
sent state of the science, and may be permitted on that ac- 
count; but in proportion as our knowledge of the mollusca 
advances, this distinction will be deemed inexpedient. In- 
deed, were this division adopted, the genera Argonauta and 
Nautilus would be torn from each other, although they are, 
by Cuvier and many others, regarded as members of a fam- 
ily of cephalopodous mollusca. The Nautilus lacustris of 
Lightfoot, now constituting the genus SEGMENTINA, would, 
in that case, likewise be separated from the genus Planor- 
bis, with which it is very closely allied. In the meantime, 
until our knowledge of the multilocular testacea arrives at 
a greater degree of perfection, such divisions may be em- 
ployed as convenient, and of easy application. 
The preceding remarks apply to those shells which belong 
to the cephalous mollusca. Among the bivalve shells, 
which belong to the acephalous mollusca, the characters 
which they exhibit are of very different degrees of import- 
ance. Here, as among the univalves, the appearance of the 
shell enables us to form an idea of the organization of the 
animal, so that the characters thus furnished by the shell Mollusca. 
a — 
may be safely employed in a natural system. 
The bivalve shells, in general, possess the faculty of moy- 
ing from one place to another, or of attaching themselves 
to rocks and stones, by means of temporary threads. These 
are termed free shells. But there are others which secrete 
at their birth a calcareous cement, which unites the shell 
to the rock or stone immoveably for life. These last are 
known by the name of jized shells. If we thus consider 
the difference in the economy of these two divisions of bi- 
valves, we may reasonably expect to find corresponding dif- 
ferences in their organization. The free shells contain ani- 
mals endowed with locomotion, and by consequence with 
feet. In few of the animals which inhabit fixed shells can 
a foot be observed. They are more: simple in their orga- 
nization than the free shells, and are destitute of absorbing or 
ejecting syphons, the place of these being supplied by holes 
in the duplicature of the cloak. This last distinction, how- 
ever, is not peculiar to the fixed shells, although found in 
all of them. 
Among the free shells, a very important circumstance 
occurs, which we have already noticed, viz. that some of 
these adhere to rocks and stones by means of temporary 
threads produced by the animal. They are termed byss¢- 
fere. Independent of the utility of this power of produc- 
ing threads of attachment, to the economy of the animals, 
the byssiferee must possess at least three organs of which 
the other testaceous mollusca are destitute. The first of 
these is a gland for the secretion of the substance of which 
the threads are formed; the second, a foot so constructed 
as to be capable of spinning these threads and fixing them 
to the rocks or other bodies to which they are intended to 
adhere ; and the third is a muscle in the animal to which 
the inner end of these threads may be attached, and which 
muscle, in general, has the power of contraction and elon- 
gation. This character, then, appears perhaps of the very 
highest order, so that, in a natural arrangement, we might 
divide the molluscous bivalves into such as spin threads of 
attachment, and such as do not. We must, however, con- 
fess, that the byssiferce have scarcely any other subordinate 
characters in common, to warrant such an arrangement. 
In general, the valves of which the shell consists close 
upon each other in such a manner as to leave no opening. 
In a few genera, however, the valves do not close upon 
each other at one end, and sometimes at both; the point 
of union being at one side or in the middle. The former 
are termed close shells, the latter gaping shells. The cha- 
racter of gaping, so very obvious in the shell, is an index 
of equally important distinctions which prevail in the ani- 
mal. In the gapers, the syphons, or the absorbing and 
ejecting pipes, are two in number, and very long, and fre- 
quently united. The foot is contained in a sheath, from 
which it issues at the pleasure of the animal. Besides, the 
branchiz are always united, and equal in length to the tubes. 
This character appears, therefore, equally important as the 
former. It has hitherto been employed in the construction 
of specific characters merely, rarely of genera. 
When the two valves are of the same size and form, the 
shell is said to be equivalve ; but when the one valve differs 
from the other in these particulars, the shell is said to be 
inequivalve. This character, so obvious and so commodi- 
ous, is not the index of any peculiar organization of the ani- 
mal. If employed in the higher divisions, it would separ- 
ate closely connected genera, and destroy some natural 
alliances. The inequivalves, however, are for the most 
part irregular in their growth. The molluscous inhabitants 
have no lengthened syphon ner foot. 
When we examine the inner surface of bivalves, we ob- 
serve some spots of a different colour and lustre from the 
general suriace. These are the places to which the muscles 
adhered, which connected the animal with the shell, and are 
