260 MARTIN F. WOODWARD. 



divisions of the Streptoneura are very differently developed, 

 and it is consequently not surprising that a considerable 

 diversity of opinion has been expressed concerning the 

 homology of the single kidney of the Monotocardia. 



A consideration of the position of the orifice of this organ 

 in the Monotocardia and its possession of a reuo-pericardial 

 pore at once suggests a comparison with the left kidney or 

 papillary sac ; but on the other hand in the position of the 

 gland itself, in the nature of its activity, and in the actual 

 presence of the peculiar renal gland, it more nearly approxi- 

 mates to the corresponding right organ of the Diotocardia. 



Ray Lankester appears to have been the first to seriously 

 attempt to seek for the homology of the single kidney of the 

 Monotocardia in the left kidney of the Diotocardia, and this 

 view, which is now practically universally accepted, has been 

 further supported by the embryological works of v. Erlanger 

 on Paludina,^ and on comparative grounds by Pelseneer. 

 Pi'actically the only opponents of this view in recent years 

 have been Haller^ and Perrier. 



The first view, which is based mainly upon the considera- 

 tion of the relation of the kidney to the rectum and the pre- 

 sence of a reno-pericardial pore, receives additional support 

 from V. Erlanger's ontogenetic observations. An examina- 

 tion of these latter shows them to be much less satisfactory 

 than one would gather from the account given in the average 

 text-book, since the only trace of the supposed missing 

 kidney, the adult right, is an angulation and faintest indi- 

 cation of an outgrowth from the pericardium (coelom) on the 

 opposite side to that at which the functional kidney is deve- 

 loping. This vestigial structure disappears very speedily 

 without ever attaining any characters which would stamp it 

 as a kidney, so that the support afforded to this theory by 

 V. Erlanger's ontogenetic researches is very meagre. A 

 similar unsatisfactory condition is attached to his surmise — 



* " Zur Entwicklung vonPaludina vivipara," 'Morpli. Jalirb.,' Bd. xvii, 

 1891. 



2 " Beilrage zur Kenntniss der Niere der Prosobraucliia," ' Morpb. 

 Jahrlj.,' Bd. xi, 18S6. 



