408 PHYLUM MOLLUSCA. 
Cephalopods, but that of Octopus has a poisonous, paralys- 
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 papillee. ; 
Vascular system.—The blood of Sega is bluish, owing 
to the presence of hemocyanin in the serum; the blood 
cells are colourless and ameeboid. 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. 
