vii MOLLUSCA RESPIRATORY ORGANS 91 



rises in the form of a series of fold-like elevations of the body wall in the pallial 



FIG. ST. Anatomy of Cavolinia 

 tridentata (after Souleyet). Shell 

 and mantle removed, and visceral 

 dome partly opened, seen from behind 

 and below. (/, Right ; s, left ; 1 , aper- 

 ture of the penis ; 2, mouth ; 3, left tin 

 (parapodium) ; 4, foot ; 5, oesophagus ; 

 6, part of the efferent genital appara- 

 tus ; 7, ventricle ; 8, auricle ; 9, herma- 

 phrodite gland ; 10, lateral processes 

 of the mantle ; 11, columellar muscle ; 

 12, intestine ; 13, digestive gland 

 (liver); 14, stomach; 15, ctenidium ; 

 16, genital aperture ; IT, anus. 



cavity, and which, running in a wavy line, forms a semicircle, open anteriorly, the 

 greater portion of it, however, lying on the right side. 



C. Lamellibranchia. 



The Lamellibranchia also possess typically two symmetrically placed gills, each 

 provided with two rows of branchial leaflets. The opinion which until lately was 

 common, that the Lamellibranchia possessed two gills on each side of the mantle 

 cavity, has been shown to be incorrect these two gills in reality answering to the 

 two rows of branchial leaflets of one typical gill. 



It is worth while to follow, step by step, the interesting series of modifications 

 undergone by the original gill in the Lamellibranchia. 



() The primitive arrangement is found in the Protobranchia. Taking Nucula 

 (Fig. 21, p. 14) as an example, we find a gill like that of Fissurella, consisting of 

 an axis along which the branchial artery and the branchial vein run, and which is 

 attached by a short membraneous band to the posterior and upper portion of the 

 body or visceral dome, and to the posterior adductor muscle. On this axis are 

 attached two rows of short flat branchial leaflets. These two plumose gills converge 

 posteriorly, and project with their free tips into the mantle cavity. The leaflets of 

 both rows are directed somewhat downwards, so that they are at right angles to one 

 another. In Malletia and Solenomya, on the contrary, they lie in the same plane, 

 the two rows standing out on opposite sides of the axis. In Malletia, this plane is 

 horizontal, but in Solenomya it trends downwards and inwards. The number of leaflets 

 on the very slender gill of Malletia is much smaller than on that of Nucula ; they 

 are consequently neither so crowded nor so flattened. Each leaflet contains a blood 



