60 



})h;iryiii;-t'al retractor which is attached to the equatui' of tlx- phalanx. It receives 

 blond h-tiiii the organs of flic iiliarynx and is prolongtMl dorsally into the canal 

 which transmits the oesophagns through the skull. A sinus, the cephalic (6^), 

 wdiich suri-(nnids the oesophageal nerve ring communicates with this canal but I 

 have not been able to locate the exact opening. This canal opens on each side 

 into tlie optic sinuses through a small foramen just behind the opening of the 

 outer into the inner buccal sinus (Plate HI Fig. 17, 28). The optic sinus (5®) 

 encloses the optic ganglion and embraces the inner surface of the eye from which 

 it receives a number of veins. These veins are grouped along the upper and 

 lower margins of the sinus where it is not overlaid by the white-bodies which 

 are wedged in between the eye and the optic ganglion. A large vein (4*) collects 

 blood from the iris and from the outer portion of the inner coat of the eye and 

 passes over the upper part of the eye with the corresponding artery to entei' 

 the optic sinus. A large vein passes from the optic sinus through the lowei- part 

 of the lateral i)late of the skull to meet a vessel from the opposite sinus. These 

 two vessels from the optic sinuses form the anterior vena cava and the wall 

 intervening between the vessels bears a pair of vertical folds which form a 

 valve that prevents the return of blood to the head. This valve is necessary in 

 order that the blood in the large venous sinuses under the visceral mass may 

 not be forced into the head by the contraction of the mantle and hence we 

 must associate it functionally with the "peripheral hearts". The valve is so effective 

 that an injection mass can seldom be forced into the head through the anterior 

 vena cava and for this reason the venous system is best injected through the 

 buccal sinuses. 



The anterior vena cava (VI) is a large , contractile median vessel situated 

 under the liver case. The ventral third of the sinus lies between the siphon and 

 the liver , its middle portion lies between the siphonal retractors upon the lower 

 surface of the visceral mass, and its dorsal portion runs above the ink sac to its 

 dorsal end and there opens into the nephridial sinus. In addition to several small 

 veins from the siphon and liver case, the anterior vena cava receives; a pair of 

 veins from the ink sac (1"), a large median vein from the Uver (2^), and a vein 

 from the salivary sinus (3"). This sinus surrounds the salivary gland, receives 

 blood from that gland and the neighboring portion of the liver, and opens into 

 the anterior vena cava between the skull and the post-cephalic cartilage. Just 

 behind this opening the anterior vena cava expands into an oval chamber whose 

 size and position correspond to the outline of the cartilage. The venous blood 

 from the two-thirds of the liver ventral to the canal for the oesophagus and the 

 aorta is collected by two median veins which run backward on the upper and 



