THE DIFFERENTIATION OF THK MESODERM-RQDIMENT, ETC. 215 



as Haliotis, Patella, FIXXH >>//<<, 7'/v>'7///x) have a second kidney. It 

 is an interesting fact that this original paired character still finds 

 expression in the development of the kidney in Pxlndiua. In the 

 adult, this kidney lies, as in most Gastropods, to the left of the 

 rectum and must therefore, as was shown above, have been the right 

 kidney before the twisting of the posterior part of the body took 

 place (Pig. 100 A-E, p. 214). This view is admirably supported by 

 v. ERLANGER'S researches, since, according to his account, it is 

 the rudiment of the right kidney which develops further, while the 

 left degenerates. P. SARASIN'S researches also show that, in 

 Bythinia, the rudiment of the kidney lies on the right side and is 

 displaced to the left later. 



The one kidney which persists in most Prosobranchs (Monotocardia) thus 

 corresponds to the (definitive) left kidney which, before the twisting took 

 place, was the right kidney of those forms which still possess two renal organs. 

 In these latter, however (Haliotix, Vissurella, Turbo, Trochus) the right kidney 

 is usually well developed, the left, on the contrary, being reduced. It thus 

 appeared possible that the permanent kidney of the Monotocardia might 

 correspond to the right kidney of the Diotocardia, a view which has been 

 put forward several times (PERBIEB, No. 87). Ontogeny, however, as well 

 as the fact that, in the Diotocardia, the right nephridium serves for con- 

 ducting to the exterior the genital products (see below, p. 220) indicate that 

 it is the left (which before torsion is the right) kidney that persists and is 

 alone retained in the Monotocardia (RAY LANKESTER, No. 65 ; v. ERLANGER, 

 No. 29). 



The pericardia! sac has several times been mentioned. The term 

 pericardia! is here hardly correct, since the kidney also originates 

 from this sac, to which, further, the heart owes its origin. This 

 organ has now become very large and has thin walls (Fig. 99). 

 Dorsally, and to the left of the renal outgrowth of the pericardium, 

 a channel-like invagination representing the rudiment of the heart 

 (Fig. 98, //) appears and occupies the whole length of the sac. The 

 channel becomes more and more marked off from the pericardium i.e., 

 it becomes a tube which at first still remains open toward the primary 

 body-cavity. This tube, by finally closing and remaining connected 

 with the wall of the pericardium only at its two ends, gives rise to 

 the heart which now, as a tube, lies within the pericardium, its two 

 ends opening into the primary body-cavity. At a somewhat earlier 

 stage, a constriction appears near the middle of this tube, by means 

 of which the auricle and ventricle are divided from one another 

 (Figs. 99 and 100). 



The vessels arise as inter-cellular spaces in the mesodermal tissue of 



