8 THE MOLLUSCA 
a hepato-pancreas, since its secretion peptonises albuminoids, converts 
starches into sugar, and saponifies fats. Absorption of digested 
food-stuffs is effected, in some forms at least, by the liver itself, 
and finally this gland has an excretory function in that it secretes 
waste products of metabolism into the alimentary tube. 
The stomach (in various Gastropods, in Scaphopods, and 
Lamellibranchs) is provided with a caecum in which a crystalline 
style is often secreted. In addition there exists, in some 
Gastropods and Cephalopods, a caecum coiled in a spiral. These 
two structures do not appear to be homologous, for in some 
Gastropods (Vassopsis) the spiral caecum and the sac containing the 
crystalline style occur together. The intestine, or at least its 
terminal portion, is furnished, in nearly all groups of Mollusca, 
with a longitudinal ridge called a typhlosole or with a furrow 
bordered by two ridges. An anal gland is present in various 
Gastropods, in Dentaliwm, and in nearly all Cephalopods. 
3. Circulation and Respiration.—In addition to the cavity of the 
alimentary tube two other important cavities, completely separated 
from one another, are found in the Molluscan body. The first, 
called the coelomic cavity, communicates freely with the exterior, 
and is generally reduced to the pericardium and the gonadial or 
genital cavity. The second is very probably the remnant of the 
blastocoel or segmentation cavity, and is continuous with spaces 
in the conjunctive mesenchyme of the integuments. It is filled 
with a fluid blood or haemolymph which is at once nutritive and 
respiratory in function. ~This cavity constitutes the circulatory 
apparatus. 
The circulatory apparatus is provided, for a greater or less 
part of its extent, with proper endothelial walls; where these are | 
absent it is lined by connective tissue so that the organs are never 
brought into direct contact with the blood. The circulatory cavity 
is, in fact, more or less specialised into arteries and veins of 
vascular structure, but there are rarely tubular capillary ramifica- 
tions, except in the integuments of Cephalopods. More usually 
the capillaries are swollen irregular cavities. The rest of the 
circulatory system is formed of sinuses ;—irregularly defined 
spaces in the connective tissue and specially abundant in the 
integuments. In fact, the phenomenon of phlebcedesis (Ray 
Lankester) is manifested in a very high degree in the Mollusca, 
the cavity of the circulatory system being distended and in- 
sinuated among the organs to such a degree as to push back 
and diminish the coelom, though no communication is ever estab- 
lished between the two. The blood-vessels pass abruptly into the: 
sinuses, and in some cases communication between sinus and blood- 
vessel is established by orifices in the walls of the latter. Remark- 
able instances of this form of communication may be seen in the 
