308 



THE CEPHALOPODA 



Tetrabranchia, has two pairs of branchiae (Fig. 276). All other 

 Cephalopoda have a single pair of branchiae, and thus constitute the 

 order Dibranchia, much richer in living species than the Tetrabranchia. 

 The branchiae are bipectinate (Fig. 272, Er\ but in some Dibranchia 

 the two sides of the organ are somewhat unequal in size. A 

 branchia is composed of lamellae whose number varies in different 

 forms, being least in the Octopoda, in which the axial branchial cavity, 

 separating the two rows of lamellae, is excessively developed. 

 Each branchial lamella is thrown into transverse folds, which are 

 in turn folded, so that the respiratory surface is largely increased. 



FIG. 274. 



Diagram showing the relations of the four nephridial sacs, the viscero-pericardial sac, and 

 the heart and large vessels in Nautilus ; ventral view. a.b.v, advehent branchial vessel ; 0.0, 

 cephalic aorta ; e.v.b, efferent branchial vessel ; neph, the two left renal sacs, each with its 

 independent aperture; r.e, glandular enlarged walls of the advehent branchial vessels (two 

 small bodies in each renal sac ; and one large body on each of the four vessels, in the viscerp- 

 pericardial sac) ; v.c, vena cava ; vent, ventricle of the heart ; visc.per.apert, arrow introduced in 

 the left aperture of the viscero-pericardial sac ; x, viscero-pericardial sac (the dotted line indi- 

 cates its backward extension, vide Fig. 270, x). (After Lankester.) 



The branchiae are not ciliated, as they are in other Mollusca, the 

 contractions of the muscular mantle sufficing to produce a current of 

 water sufficient for respiration. The rate of the respiratory move- 

 ments of the mantle is variable, and is generally quicker in the 

 Decapoda than in the Octopoda. 



In Nautilus (Fig. 276) the branchiae are free throughout their 

 extent, but in the Dibranchia they are attached dorsally to the 

 mantle by their afferent borders (Fig. 272). A special glandular 

 organ, whose function is not exactly known, is situated along the 

 line of attachment : it receives the blood which, having circulated 

 through the nutrient vessels of the gill, has to pass through the 

 kidney along with the venous blood from the mantle, to be returned 



