64 



the anus. This papilla gives oft' 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 ti) 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 ves.sels. 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 tlows through the 

 capillaries toward the centei- of the fold and into the tertiary vein at the middle 

 of each fold. 



The gill Anally 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 

 arter}' and vein , \vhich bear a lai-ge number of alternating pinnae. The latter are 

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

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

 pinnae and the thiixl . f(U'med by the branchial vein . is directed towaixl the 

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

 communicates with the pallial chamber between the pinnae. On the outei- side 

 of the artery the branchial nei've cord and a vein of the systemic circulation lun 

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

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

 base of the gill partly to the mantle and partl>- 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 uii 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 nf the pinna. 

 The free edge of this membrane is stiftened by an acicular i-od 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 I'od 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 



