50 



COMPARATIVE ANATOMY 



CHAP. 



An anal aperture, separated by a point of concrescence from the large mantle cleft, 

 is found in the following Lamellibranchia : 



(a) Among the Protobranchia in Mallet in. 



(b) Among the Filibranchia in most Mytilidce. 



(c) Among the Pseudolamellibranchia in the Aviculidce (genus Mcleagrinci). 



(d) Among the Eulamellibranchia, in the C'ardituhe (I'cncrn'n ,</;</, Cardita 

 Miliicria}, the Astartidce, and most Grassate llidcc ; among the Cyrcnidcc, in the genus 

 Pisidium, ; among the Unioiiidce in the Uni<niina: (U/tio, Anodonta) ; and among 

 the Lucinacca, in Cryptodon *}[uleyi. 



In Sole iwni i/a, among the Protobi-micliin. the two mantle edges grow together 

 only at one point, but to such an extent as to close the whole posterior half of the 



FIG. 57. Diagrams to illustrate the various ways in which concrescence of the mantle 

 and formation of siphons take place in the Lamellibranchia. The foot (7) protruded forward 

 through the mantle cleft ; A, mantle completely open ; B, mantle open, but with its edges applied 

 to one another at two points, thus giving rise to incompletely separated anal and respiratory 

 cavities ; C, edges of the mantle grown together at one point (1), the anal or exhalent aperture of 

 tin- mantle (4) is separated ; D, edges grown together at two points (1, 2), the branchial or iulialent 

 aperture (5) is also separated, the mantle has three apertures ; E, mantle closed by the extension 

 of the place of concrescence (2), three limited apertures remain, viz. the anal, branchial, and pedal 

 apertures the first two are produced into siphons ; F, a third concrescence (3) takes place. 

 Mantle with four apertures (4, 5, Ga, Gb), the most anterior (6b) for the protrusion of the foot. The 

 siphons have united. 



ventral mantle opening. In this way the mantle cleft is divided into two ; the anterior 

 aperture serves for the protrusion of the foot, while the posterior serves at the same 

 time as inhalent (branchial) and exhalent (anal) aperture. Solaioiiiya is the only 

 1 livalve in which this arrangement is found. 



C. The mantle folds grow together at two points, thus forming three 

 apertures. This condition arises in consequence of the complete separation (through 

 concrescence) of the branchial aperture from the rest of the large anterior mantle 

 cleft. The anal and branchial apertures may remain as slits, or may be produced 

 into longer or shorter anal and branchial siphons. The large anterior and ventral 

 mantle cleft serves for the protrusion of the foot, and is called the pedal cleft. 

 These two points of concrescence are found : 



(a) Among the Protolraitchin, in Yoldia and Lain. 



