254 



COMPARATIVE ANATOMY 



CHAP. 



communicate with the body cavity. The heart tube becomes constricted in the 

 middle, the anterior division forming the auricle and the beginning of the branchial 

 vein, the posterior, the ventricle and the rudiment of the body aorta. 



Fig. 210. — Development of Paludlna vivipara (after v. Erlangor) A, Right aspect of an 

 embryo, in which the pericardium Is divided into two parts by a septum. B, The same of a some- 

 what older embryo, with an undivided pericardium. C, The same of an older embryo, in which the 

 first rudiment of the heart has appeared. D, Ventral aspect of the posterior end of an embryo, in 

 which the asymmetry of the visceral dome begins to appear. The anus is still median, but the 

 mantle cavity is already deeper on the right (the left in the figure). 1, Velum ; 2, raid-gut ; 3, 

 digestive gland (liver) ; 4, pericardium ; 4a and 4&, divisions of the same formed by a septum ; 5, 

 free edge of the shell ; 6, shell groove ; 7, anus ; 8, mantle cavity ; S5, base of the mantle cavity = 

 base of the mantle fold ; 9, free edge of the mantle ; 10, foot ; 11, auditory organ ; 12, oesophagus ; 

 13, cephalic tentacle ; 14, eye ; 15, efferent duct of the (originally) right nephridiura ; 15?j, rudi- 

 mentary efferent duct of the (originally) left nephridium ; 16, primitive kidney ; 17, rudiment of 

 the heart ; 18a, right nephridium ; 18&, rudimentary left nephridium. 



Fig. 211 A shows a somewhat older embryo which already resembles in form the 

 adult animal. The velum is reduced, and a ventral bulging of the anterior division 

 of the oesophagus represents the rudiment of the radular sac. The ventricle and 



