308 LHE CEPHALOPODA 
Tetrabranchia, has two pairs of branchiae (Fig. 276). All other 
Cephalopods 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, br), 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. 
Fic. 274. 
Diagram showing the relations of the four nephridial sacs, the viscero-pericardial sac, and 
the heart and large vessels in Nawtilus; veutral view. a.b.v, advehent branchial vessel ; a.o, 
cephalic aorta; e.v.b, efferent branchial vessel; neph, the two left renal sacs, each with its 
independent aperture; 7.e, glardular 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 viscero- 
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 ; 2, viscero-pericardial sac (the dotted line indi- 
cates its backward extension, vide Fig. 270, 7). (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 
