THE MOLLUSC A 



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 (Nassopsis) 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 Dentalium, 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 diret-, 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 



