PHYLUM MOLLUSCA 



765 



into sugar; they sometimes, though without sufficient reason, 

 receive the name of pancreas. 



Immediately below the thin integument of the anterior wall of 

 the mantle-cavity lies a characteristic organ the ink-sac (Fig. 

 658, ink. s. ; Fig. 661, i). This is a pear-shaped body, a portion of 

 the interior of which is glandular and secretes a black substance 

 the ink or sepia which collects in the main cavity of the sac and 

 is discharged by a cylindrical duct 

 opening into the rectum close to 

 the anal aperture. When the 

 Cuttle-fish is startled it discharges 

 the ink, which, mixing with the 

 water in the mantle-cavity, is 

 ejected through the funnel as a 

 black cloud, under cover of which 

 the animal may escape from a 

 threatened attack. 



Vascular System. The heart 

 (Figs. 662, 663, and 665) of the 

 Cuttle-fish consists of a ventricle 

 and two auricles. The ventricle 

 (vent.) which is divided into two 

 lobes by a constriction, is some- 

 what obliquely placed, but the 

 rest of the vascular system is 

 almost completely equilateral. At 

 its oral end the ventricle gives 

 off a large vessel the oral aorta 

 (aort.); aborally it gives origin 

 to be a much smaller aboral 

 aorta (aort'), which bends over 

 the ink-sac and supplies the 

 aboral portions of the body. The 

 arteries which lead off from the 

 aortae communicate by their 

 ultimate branches with a system 

 of capillaries, and these with a 



system of veins. A large median vein, the vena cava (c. cav.), runs 

 from the head to the neighbourhood of the rectum, in front of 

 which it bifurcates to form the left and right afferent branchial 

 veins (/. aff. br. v., r. aff. or. v.), each running through the cavity of 

 the corresponding renal organ to the base of the gill, where it is 

 joined by veins from the aboral region. At the base of the 

 gill the afferent branchial vein becomes dilated to form a con- 

 tractile sac the branchial heart (r. br. ht] appended to which 

 is a rounded body of a glandular character the appendage 

 of the branchial heart, representing the pericardial glands of the 



VOL. i 3 c 



FIG. .".'.--Sepia officinalis, jaws. A, 

 in situ ; B, removed and slightly en- 

 larged. (From the Cambridge Natural 



