346 



INVEBTEBBATE MOBPEOLOGT. 



veins return tlie 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 Oephalopods 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. 



Fig. 154. — Circulatory Apparatus of Sepia (after Hunter from Owen).. 

 ao = anterior aorta. Iv = lateral vein. 



ad = abdominal aorta. ne = excretory appendage. 



au = auricle. 

 bh = branchial heart. 

 et = ctenidium. 



pg = pericardial gland. 



« = ventricle. 

 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 {au) which re- 

 ceive the blood from the branchire {ct). Two principal arte- 

 ries arise from the ventricle, a larger one running ventrallv 

 (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 



