I 68 RELATIONS BETWEEN BRANCHIAE AND HEART chap. 



tion of the phenomena of a single group. Taking the Septi- 

 branchiata as an instance, we find that in Lyonsiella the branchiae 

 unite with the mantle in such a way as to form two large pallial 

 chambers, the structure of the branchiae being preserved, and 

 their lamellae covering the partition. A further stage is 

 observed in Poromya. There, a similar partition exists, but it 

 has become muscular, preserving, however, on each side two 

 groups of branchial lamellae, separated one from the other by a 

 series of slits, which form a communication between the two 

 pallial chambers. A further stage still is seen in Silenia. There 

 the same muscular partition exists, but the branchial lamellae on 

 either side have disappeared, the slits between the two chambers, 

 which occur in Poromya, still persisting, but separated into 

 three groups. Cuspidaria represents the last stage in the 

 development. In the ventral chamber there appears nothing at 

 all corresponding to a branchia ; the surface of the partition 

 appears perfectly uniform, but on careful examination three little 

 separate orifices, remains of the three groups of orifices in Silenia, 

 are observed.^ 



Relation between Branchiae and Heart. — The object of the 

 branchiae being, as has been already stated, to aerate the blood 

 on its way to the heart, we find that the heart and the branchiae 

 stand in very important structural relations to one another. 

 When the branchiae are in pairs, we find that the auricles of the 

 heart are also paired, the amicle on the right and left sides being 

 supplied by the right and left branchiae respectively. This is 

 the case with the Dibranchiate Cephalopods (Argonauta, Octopus, 

 Loligo, etc.), the Zygobranchiate Prosobranchs {Pissurella, Hali- 

 otis), and all Pelecypoda. In the Amphineura {Chiton, etc.) there 

 are two auricles corresponding to the two sets of multiple 

 branchiae. In the case of the Tetrabranchiate Cephalopods 

 {Nautilus) there are four auricles corresponding to each of the 

 four branchiae. Compare Fig. 79, A, B, C, D, E. 



On the other hand, when the branchia is single, or when both 

 branchiae are on the same side, and one is aborted and functionless, 

 the auricle is single too, and on the same side as the branchia. 

 This is the case with the Tectibranchiate Opisthobranchs {Philine, 

 Scaphander, etc.), all the Pectinibranchiate Prosobranchs (Eachi- 

 glossa, Taenioglossa, and Ptenoglossa), and the other Azygobran- 

 ^ Pelseneer, Comptes Rendus, cvi. p. 1029. 



