462 PHYLUM MOLLUSCA 



ing effect on the crabs, etc., which are bitten, and also a 

 peptonising action. At the other end of the food canal, 

 the ink-sac, full of black pigment, probably of the nature 

 of waste products, opens into the rectum close to the anus. 

 This ink-sac is a much enlarged anal gland ; for, while 

 most of the bag is made of connective tissue and some 

 muscle fibres, a distinct gland is constricted off at the 

 closed end, and the neck is also glandular. Beside the 

 anus are two pointed papillae. 



Vascular system. — The blood of Sepia is bluish, owing 

 to the presence of hasmocyanin in the serum ; the blood 

 cells are colourless and amoeboid. The median but some- 

 what oblique ventricle of the heart drives the blood forward 

 and backward to all parts of the body. It reaches the 

 tissues by capillaries, and apparently also by lacunar spaces. 

 The venous blood of the head region is collected in an 

 annular sinus round the basis of the arms, and passes 

 towards the heart by a large vena cava, which divides into 

 two branchial veins, covered by spongy outgrowths of the 

 nephridia. Joined by other vessels from the apical region 

 of the viscera, each branchial vein enters a " branchial 

 heart " at the base of each gill. The branchial heart is 

 contractile, and drives the venous blood through the gills, 

 whence, purified, it returns by two contractile auricles into 

 the ventricle. There are valves preventing back-flow from 

 the ventricle to the auricles, or from the arteries to the 

 ventricle. Beside each branchial heart lies an enigmatical 

 glandular structure known as a " pericardial gland," 

 possibly an excretory or incipiently excretory organ. The 

 course of the blood differs from that in the mussel and 

 snail in this, that none returns to the heart except from the 

 respiratory organs. In the nephridial outgrowths around 

 the branchial veins the interesting parasite Dicyema is 

 found. 



Respiratory system. — The blood is purified by being 

 exposed on the two feather-like gills which are attached 

 within the water-washed mantle cavity. The water pene- 

 trates them very thoroughly ; the course of the blood is 

 intricate. At the base of the gills there is some glandular 

 tissue, which those impatient with enigmas have credited 

 with blood-making powers. 



