340 



COMPARATIVE ANATOMY. 



Cephalopoda. Iu these latter tlie gills arise in just the same way — ■ 

 between the mantle and the foot (Fig. 171, B br) — as they do per- 

 manently in many Gastropoda. When, however, the mantle is 

 developed they sink downwards, and are then placed in a mantle- 

 cavity, which, as compared with the Gastropoda, appears to open on 

 the hinder surface. The gills are arranged symmetrically in all of 



.V.lj7 



Fig. 178. Mantle-cavity and funnel of Sepia officinalis. The mantle-cavity has 

 been opened by an incision along the middle line. In it is seen the visceral sac 

 projecting, while posteriorly to it two muscular branches are given off (m.) to the 

 funnel and head. Br Branchiae, v br Branchial vein, v br' Its bulbous enlargement. 

 t Ink-bag. r Orifice of the excretory organ, opened on the right side, and displaj'ing 

 at R the venous appendage, g Genital papilla, a Anus. J Funnel, opened by an in- 

 cision along the middle line, i Tongue-shaped organ, c Depression for the process at 

 the edge of the mantle (paHial hinge) c'. C Head. P Fins. 



them ; there are two pairs in the Nautilus, but only one in all the 

 other extant Cephalopoda. 



As a rule, each gill is pyramidal in form, with the apex directed 

 towards the side, and its base towards the middle Hue (Fig. 178, Br). 

 It either consists of closely-approximated lamella 1 , which gradually 

 increase in number at the tip (Nautilus, and most Loliginidse), or 

 of several much-coiled dermal folds, which arise between the two 

 branchial vessels which pass to the edge of the mantle (Octopoda). 



