12 THE MOLLUSCA 
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 
“Jung.” 
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-pericardial 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 
