THE MOLLUSCA II 
the name of the “respiratory apparatus.” ‘It is constituted 
by the ctenidia or branchiae properly so-called, of which there 
may be one or many pairs. There are two pairs in Nautilus ; 
from four to eighty pairs in the Polyplacophora, and where a 
single pair is normally present it may be reduced to a single 
azygos organ, generally in correlation with the reduction of 
the auricles. 
The ctenidia are situated primitively in the posterior or anal 
region of the mantle, but they may be multiplied and spread 
anteriorly, or both anteriorly and posteriorly (Polyplacophora, 
Fig. 28), or without being multiplied they may extend pro- 
gressively towards the region opposite to their primitive situation, 
Diagrams of transverse sections of the ctenidia of various Mollusca. I, Chiton; II, 
Pleurotomaria ; 111, Trochus; 1V, Nucula; V, Nautilus; VI, Chaetoderma ; VII, Haliotis; VLII, 
Lacuna ; 1X, Solenomya; X, Sepia. a, afferent vessel ; e, efferent vessel ; pa, mantle. 
as in Gastropods and Lamellibranchs. They are shorter in Nucula 
than in Arca; shorter in Arca (Fig. 188) than in Avicula (Fig. 
236); shorter in Pleurotomaria than in Trochus, and in Trochus than 
in Fissurella. 
These ctenidia have exactly the same structure in the archaic 
members of the different groups: an identical fundamental 
structure may be recognised in the Polyplacophora, in the 
Rhipidoglossa among Gastropods, in the Protobranchs among 
Lamellibranchs, and in the Cephalopods (Fig. 5). Each etenidium 
consists of an axis containing two vascular trunks. The one, 
an afferent vessel, in which the blood current is centrifugal, 
communicates with a “vena cava” or with a simple venous sinus ; 
the other is the efferent vessel, in which the current is centripetal, 
and the auricle is nothing more than its specialised terminal 
portion. The auricle, in fact, has the innervation of a pallial 
