76 



LAMELLIBKANCHIA. 



trated in the embryo of Cyclas (Fig. 31) and is retained in the adult in the 

 more compact forms such as Pect-en and Cardium, is modified in so far as it 

 extends beneath the pericardium almost to the anterior end of the ventricle. 

 Its opening into the pericardium then lies in this region. From this point 

 the renal sac runs back to the posterior adductor ; the organ then again bends 

 upward into the renal coil which is formed of three consecutive chambers 

 (RANKIN). These, as well as the renal sac, are richly provided with folds, 

 which project far into the lumen. From the last chamber of the renal coil, 

 the renal duct runs forward parallel with and above the renal sac. Towards 

 the anterior end, the two efferent renal ducts communicate with one another 

 (inter-renal aperture) underneath the pericardium. Near the anterior end of 

 each renal duct, a short ureter passes outward, its aperture lying at the base of 

 the inner gill. 



Our knowledge of the ontogeny of the kidney in the Unionidae is limited to 

 the fact that it also arises in the form of two tubes out of the mesodermal 

 tissue at the base of the gill-rudiment (F. SCHMIDT). These tubes, as in 

 Cyclas, form several coils and no doubt extend later, in the way described in 

 connection with the adult, beneath the pericardium. Near the rudiment of 

 the kidney, and also at the base of the gill, the pericardium arises as a cavity 

 in the mesodermal mass and here also seems early to become connected with 

 the kidney. 



The Formation of the Heart. In following the development of the 

 organ of Bojanus, we left the pericardial vesicles temporarily out of 



consideration, al- 

 though they also 

 underwent essen- 

 tial modifications 

 of form. After 

 the vesicles have 

 slightly lengthened, 

 they become partly 

 constricted, the 

 outer wall becoming 

 invaginated (Fig. 

 32 A, >). In this 

 way, each vesicle 

 appears to be 

 divided into two, 

 but the division is 

 not complete and 

 the two halves of 

 the vesicle still 

 communicate with one another. In the ventral wall of the lower 

 section, the internal opening of the kidney can be recognised (Fig. 



FIG. 32. Diagram of the formation of the heart in Cyclas 

 (constructed from ZIEGLER'S descriptions). , auricle (in 

 B, the pericardial invagination which leads to its forma- 

 tion) ; d, intestine ; g, vessels opening into the auricles ; 

 n, renal funnel ; p, pericardium ; v, ventricle. 



