THE MOLLUSC A 



ii 



the name of the "respiratory apparatus." It is constituted 

 by the ctenidia or branchiae properly so-called, of Avhich 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, 



Fio. 5. 



Diagrams of transverse sections of the ctenidia of various Mollusca. I, Chiton ', II, 

 Pleurottnnaria ; III, Trochits; lV,Nucula; V, A'awfthw ; VI, Chcutoderna ; VI I, HoHotis; VIII, 

 lMc-u.no, ; IX, Soieaomga ; X, Sepia, a, afferent vessel ; e, efferent vessel ; pu, mantle. 



as in Gastropods and Lamellibranchs. They are shorter in Nucula 

 than in Area; shorter in Area (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 ctenidium 

 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 



