64 



the anus. This papilla gives off alternate buds from its tip and each bud soon 

 forms a triangular sheet whose inner and outer margins are formed by a second- 

 ary branchial vein and artery. The surface of this sheet grows much more 

 rapidly than the vessels and so it is thrown into folds whose axes are perpen- 

 dicular to the vessels. A tertiary artery forms in every angle of the folds and a 

 tertiary vein develops in the middle of each fold. The surface continues to grow 

 more rapidly than the \'essels and is thrown into a second series of folds whose 

 axes are perpendicular to those of the first fold. The capillaries of the gill lie in 

 the folds between the tertiary vessels. The blood passes from the secondary artery 

 at the outer edge of the pinna — i. e. the edge towards the mantle — into the tertiary 

 arteries at the edges of the primary folds and from them flows through the 

 capillaries toward the center of the fold and into the tertiary vein at the middle 

 of each fold. 



The gill finally becomes a long pinnate structure which lies beside the liver 

 in the pallial chamber and extends from the heart to the entrance of the siphon. 

 The water entering the mantle over the lateral siphonal valve passes over and 

 through the gill. The gill proper consists of two longitudinal vessels, the branchial 

 artery and vein , which bear a large number of alternating pinnae. The latter are 

 moulded over a muscular and glandular core so that the giU is trihedral. The 

 two angles towards the mantle are formed by the ends of the two rows of 

 pinnae and the third , formed by the branchial vein , is directed toward the 

 liver. The branchial vein is separated from the artery by a water canal which 

 communicates with the pallial chamber between the pinnae. On the outer side 

 of the artery the branchial nerve cord and a vein of the sj'^stemic circulation run 

 the length of the gill. The branchial gland, which is enclosed in a sheath of 

 fascia , lies between the nerve and the mantle. Bands of muscle attached at the 

 base of the gill partly to the mantle and partly to the visceral mass lie above 

 and below the nerve , vein , and gland. They form one of the chief connections 

 between the mantle and the viscera. A thin extensile sheet of integument 

 attaches the gill to the mantle. This sheet reaches almost to the tip of the gill 

 and is necessary to hold the gill up against the water rushing into the pallial cavity. 



Each pinna is attached to the core of the gill by a triangular sheet of 

 fascia which is fastened to the secondary artery at the lower edge of the pinna. 

 The free edge of this membrane is stiffened by an acicular rod of chitinous 

 nature whose broadened base is attached to the side of the gland and whose 

 apex is fastened to the tip of the pinna. The rod is secreted by a tube of 

 cylindrical cells. Burne finds in Sepia that these rods are formed of typical cartilage. 



The gill grows continuously at the tip and at the ends of the pinnae so 



