346 



INVERTEBRATE MORPHOLOGY. 



veins return the blood carried by the arteries to various por- 

 tions of the body, definite capillaries connecting the two sets 

 of vessels. A lacunar system also exists, however, so that, 

 while showing a much greater differentiation than the other 

 Mollusca, the Cephalopods yet retain indications of the more 

 primitive arrangement. 



The heart consists of a tubular ventricle (Fig. 154, v), 

 usually arranged with its long axis directed dorso-ventrally, 



va 



FIG. 154. CIRCULATORY APPARATUS OP Sepia (after HUNTER from OWEN). 

 ao = anterior aorta. Iv lateral vein. 



ao' -- abdominal aorta. ne excretory appendage. 



au = auricle. pg = pericardial gland. 



bh = branchial heart. v = ventricle. 



ct = ctenidium. va abdominal vein. 



vc = cephalic vein. 



though in the Octopoda it is transverse, and has opening into 

 it at each side one or two (Nautilus') auricles (ati) which re- 

 ceive the blood from the branchiae (ct). Two principal arte- 

 ries arise from the ventricle, a larger one running ventrally 

 (ao), and a smaller one which runs towards the tip of the 

 visceral hump and supplies the viscera of that region (ao'). 

 As already stated, these arteries pass into a fine capillary net- 

 work from which the veins arise, sinuses, however, interven- 

 ing in some cases in the course of the latter, and possibly 

 some arterial branches may terminate in such sinuses. The 



