50 



COMPARATIVE ANATOMY 



An anal aperture, se})arated by a point of concrescence from the large mantle cleft, 

 is found in the following Lamellibranchia : 



(a) Among the Prutobranchia in Malletia. 



(b) Among the Filibranchia in most Mytilidoe. 



(c) Among the Pseudolaniellibranchia in the AvicuUdce (genus Mdeagrina). 



(d) Among the Eulamcllibranchia, in the Carditidce {Vencricardia, Cardita 

 Milneria), the Astartidcc, and most Crassatellidce ; among the Cyrenidce, in the genus 

 Pisidium ; among the Unionidoc in the Unimiince [Uitio, Anodonta) ; and among 

 the Lucinacea, in Onjptodou Moselcyi. 



In Solenomya, among the Protobranchia, 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 

 tlirougli the mantle elett ; 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 exlialent aperture of 

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

 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, 6a, 6b), the most anterior (66) 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. Solenomya is the only 

 bivalve 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 : 



(«) Among the Protobranchia, in Yoldia and Lcda. 



