12 THE MOLLUSC A 



organ like the ctenidium ; the ventricle that of a visceral organ 

 properly so called. Each side of the axis bears a row of respiratory 

 filaments, generally flattened, but of variable shape, whose cavities 

 communicate with the two vascular trunks of the axis. It is in 

 the cavities of these filaments that the blood absorbs oxygen 

 dissolved in the water. The continuous renewal of water on the 

 surface of the ctenidium is provided for by a covering of ciliated 

 epithelium. The ciliated epithelium is absent in Cephalopods, but 

 in this group the powerful musculature of the mantle and siphon is 

 sufficient for the purpose. 



The whole volume of venous blood, however, is not in all cases 

 passed through the ctenidia : a smaller or larger part may be 

 distributed to the mantle and thence returned directly to the 

 heart. This arrangement is found in a considerable number of 

 Gastropods (Heteropods, Pleurobranchs, and Nudibranchs) and in 

 the majority of Lamellibranchs. Finally, the typical respiratory 

 apparatus may be complicated by specialisation or by reduction, 

 and may disappear altogether, as in the Neomemiidae, the 

 Scaphopoda, the Septibranchia, and a large number of Gastropoda. 

 In such cases the function of oxygenating the blood is wholly 

 transferred to the free surfaces of the pallial integuments, which 

 often form a secondary respiratory organ, especially in the 

 Gastropoda. In aquatic species this secondary apparatus takes 

 the form of "pallial branchiae," in terrestrial species of a 

 "lung." 



In certain cases there is a localised blood-gland or lymphatic 

 gland which, from its phagocytic function and the formation of 

 amoebocytes, may be said to have the physiological character of 

 the spleen of Vertebrates. This organ is generally situated on the 

 course of the aorta, instances being found in many Opisthobranchs 

 and in the "white body" of Cephalopods (Fig. 268). It consists 

 of conjunctive tissue in which blood corpuscles are formed at the 

 expense of the conjunctive cells. In other cases the gland is 

 diffuse, that is to say, distributed in a more or less irregular fashion 

 in the conjunctive tissue in the form of plasmatic cells. 



4. Coelom. The walls of the coelom of Molluscs are completely 

 covered by a continuous epithelium, partly genital, partly excretory. 

 The coelom is divided into several different cavities, gonadial, 

 pericardial, and renal, the two last named being excretory. The 

 gonadial and reno-pericardiul cavities are separate, except in the 

 Cephalopods and aplacophorous Amphineura. It must be assumed 

 that the " Prorhipidoglossa," the common ancestors of the 

 Gastropods, Scaphopods, and Lamellibranchs, had also this 

 communication between the pericardium and genital cavity, and it 

 is preserved in the more archaic Gastropods and Lamellibranchs 

 (certain Rhipidoglossa and Protobranchs), in which the gonads 



