86 COMPARATIVE ANATOMY chap. 



With regard to the number of gills originally present on each side 

 of the body, opinions are divided. Those who hold that there were 

 several seem justified by the arrangement in Chiton, where numerous 

 consecutive ctenidia lie in a longitudinal row in the branchial furrow 

 (mantle cavity) on each side, and also by that in the Nautilus, which 

 is rightly considered the most primitive of extant Gephalopods, where 

 four gills are found (Tetrabranchia). We shall, however, see later that 

 the other view, viz. that the Mollusca originally possessed only one 

 pair of ctenidia, has, to say the least, equal claim to be accepted. 



In all other Mollusca with paired ctenidia, including the Lamelli- 

 branchia, there is only one pair at the posterior part of the body. 

 Further, in the racial form of the Prosobranchia, a single pair of gills 

 must be assumed to have occupied a posterior position in a mantle 

 cavity which, with them, shifted forward later to the anterior position. 

 The Zeugobranchia still retain this single pair of gills. 



In most Prosobranchia, the asymmetry of the body is also seen in 

 the gills, only the left gill of the two in the Fissurellidce and Haliotidce 

 being retained, the right completely disappearing. In the forms which 

 most resemble the Fissurellidm and Haliotidm, the single-gilled Dioto- 

 cardia (Turbinidm, Trochidce, etc.), the gill is still feathered on both 

 sides, but in all Monotocardia it has only a single row of leaflets. 



In one division of the Opisthobranchia, the Tectibranchia, one 

 ctenidium is still retained, that on the right side. Other Opistho- 

 branchia have lost the true ctenidium together with the mantle cavity ; 

 it may be/replaced by analogous (but not homologous) respiratory 

 organs, such as adaptive gills. 



The Ptdmonata, in consequence of their adaptation to aerial 

 respiration, have lost the ctenidia. 



The blood, which has become arterial in the ctenidia, reaches the 

 heart through the auricle, and passes into the body through the 

 arteries. It is therefore evident that a close relation must exist 

 between the gills and auricles. This relation is briefly as follows : 

 where the gills are paired, the auricles are paired, and unpaired gills 

 are accompanied by a single auricle on that side of the body on which 

 the gill is retained. Where gills are paired, there is almost always 

 only one pair, and then there is one right and one left auricle. 



The Nautilus has four gills, and, to correspond, two right and two 

 left auricles. The Ghitonidce, on the other hand, in spite of their 

 numerous pairs of gills, have only one right and one left auricle. 



The Scaphopoda possess neither true ctenidia nor any other localised 

 gills. Respiration may take place at the various soft-skinned surfaces 

 which come in contact with the water, such as the inner surface of the 

 mantle, the tentacles, etc. 



A. Araphineura. 



Chitonidse. — A single ctenidium of a Chiton (Fig. 82) may serve as a type of the 

 Molluscan gill with its two rows of leaflets. The plumose ctenidium rises freely from 



