EXCRETORY OEGAI^S OF MOLLUSCA. 375 



The ca3lom is sucli a blood-space ; all the organs in it are bathed by- 

 venous blood. Various veins open into it, and it is also connected 

 by two canals with the large vena cava (vena cephalica). 



Milne-Edwards et Valenciennes, Nouv. obs. sur la constat, de I'appareil de la 

 circulation chez lea Moll. Mem. Acad, des Sc. T. XX. Milne-Edwards, 

 Voyage en Sicile. T. I. 



§ 288. 



The blood fluid of the Mollusca is, as a rule, colourless, and 

 often has a bluish or opalescent appearance. In many Cephalopoda, 

 however, it is violet or green, and in some Gastropoda (Planorbis) 

 the blood is red — the plasma being coloured. 



The form-elements of the blood are always colourless, and have 

 the character of indifferent cells, which give off all kinds of pseudo- 

 podia-like processes dui-ing their amoeboid movements; this has 

 been observed in the Lamellibranchiata and Gastropoda. 



A rounded organ extends alongside the branchial arteries of the 

 Cephalopoda ; its function is not known, but it may perhaps be an 

 organ which is of importance in the development of the form- 

 elements of the blood. 



[Lankester, E. Eat, Distribution of Haemoglobin in the Animal Elingdom 

 (red corpuscles of Area and Solen). Proc. Koyal Society, 1873.] 



Excretory Organs. 



§ 289. 



In addition to the various organs which have been already noted 

 as present in the integument, and which serve as excretory organs, 

 there are others which open on to the surface of the body, and 

 which are much more important from this point of view. 



In the Placophora there is a glandular organ which is placed 

 close to the anus ; but it is not certain that it is comparable to the 

 excretory organ of the Conchifera. Its internal orifices have not 

 been observed. For the present, therefore, we must regard this 

 organ as not belonging to the same series as the excretory organs of 

 the rest of the Mollusca. 



These typical excretory organs of the Mollusca are 

 homologous with the organs found throughout the Vermes, 

 and there called renal; which organs form the looped 

 canals (nephridia) of the Annulata, In the Mollusca also they com^ 

 mencebyan external orifice, and open into the coelom after a longer 

 or shorter course. The internal opening is distinguished by special 

 arrangements; most commonly, perhaps, indeed, generally, by an in- 

 vestment of cilia, whereby they call to mind the ciliated funnels of the 

 looped canals of Vermes. Owing to the presence of these organs, the 

 internal cavity of the body communicates with the surrounding 

 medium. They are able, therefore, to bring water into the body, 

 while, like their homologues among the Vermes, they may preside 



