EXCEETOEY OEGANS OF MOLLUSCA. 



379 



§ 292. 



The great variations in the more special characters of the excre- 

 tory organ, which are seen in the Gastropoda, prevent our being 

 astonished at the fact that this same organ is otherwise modified in 

 the Cephalopoda. All the Cephalopoda possess in their visceral mass 

 closed sacs, which open into the mantle-cavity. As the efferent ducts 

 for the generative products resemble the excretoiy canals in having 

 that part of them which surrounds the germ - glands connected 

 with the coelom, it is pro- 

 bable that these efferent 

 dacts have been derived 

 from primitive excretory 

 organs ; in this case the 

 Cephalopoda must have 

 had a larger number of 

 these organs, of which 

 some alone have retained 

 their primitive significance. 

 Four of these organs are 

 found in Nautilus, and two 

 in the Dibranchiata. Their 

 orifice is sometimes placed 

 on a papilliform process 

 (Fig. 178, r). The large 

 branchial vascular trunks 

 project into these sacs, and 

 their walls, therefore, are 

 irregular in form. So far, 

 however, as these vessels 

 project into the sacs, their 

 walls must be regarded as 

 belonging: to the sac. In 

 connection with the bran- 

 chial arteries the wall of 

 each sac presents numerous 

 ramified appendages, which project into the lumen of the sacs (cf. 

 Fig. 178, li; Fig. 202, re) ; they are formed by ctecal diverticula of 

 the vessel, and a superjacent glandular investment. In Nautilus these 

 appendages of the four venous trunks are clothed by tubular glands 

 which open into the connected sac. It is still doubtful what we 

 should regard the appendages on the other blood vessels, which 

 project into the pericardial sinus, as being. As this sinus communi- 

 cates with the mantle-cavity, perhaps they also represent excretory 

 organs. In the Dibranchiata the venous appendages appear to 

 have a somewhat different structure. Concretions of phosphate 

 of calcium appear to be the principal products of this appa- 

 ratus ; in the Sepiadae this organ (Fig. 201) is very large, and 



Fig. 202. Circulatory and excretory organs of 

 Sepia, hr Branchiee. c Heart, a Anterior 

 artery of the body (aorta), a' Posterior artery. 

 V Enlargements of the branchial veins, repre- 

 senting the auricles of the heart, v' Branchial 

 vein, running alongside the branchia). re An- 

 terior large vena cava, vc' Branchial arteries 

 (branches of the vena cava). vc" Posterior 

 vena3 cavse. re Spongy appendages of the 

 branches of the vcna3 cava3. x Their diverticula. 

 The arrows indicate the direction of the blood- 

 current (after J. Hunter.) 



