1 68 RELATIONS BETWEEN BRANCHIAE AND HEART chap. 



of the phenomena of a single group. Taking the Septibranchiata 

 as an instance, we find that in Lyo7isiella 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. Ciispidaria represents the last stage in the develop- 

 ment. 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 hetiveen Branchiae ayid 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 tlie auricles of 

 the heart are also paired, the auricle 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 (^Fis- 

 surella, Haliotis)^ 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 (^IVautilus') 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 function- 

 less, the auricle is single too, and on the same side as the 

 branchia. This is the case with the Tectibranchiate Opistho- 

 branchs (^Philine, Scaphander^ etc.), all the Pectinibranchiate 

 Prosobranchs (Rachiglossa, Taenioglossa, and Ptenoglossa), and 

 1 Pelseneer, Comptes Bendus, cvi. p. 1029. • 



