TOPOGEAPHICAL EELATIONS AMONG THE DOCOGLOSSA, 



273 



large, and forms the papillary sac, lying practically in the mantle-roof in front of the 

 pericardium ; it seems to possess a pericardial canal in these forms also. Lankester and 

 Pelseneer have therefore homologized the left kidney of these Rbipidoglossa with the single 

 kidney of Monotocards, and their view has been supported by the ontogenetical work of 

 Erlanger, which was, however, confined to the rather specialized Faludina. The difficulties 

 in the way of this view have been well urged by Perrier, who found that the papillary 

 sac is no longer an organ for the excretion of nitrogenous waste, but, quite on the other 

 hand, a builder of reserve material. Another of his arguments was that the left kidney 

 of these forms is represented by the nephridial gland of the Monotocards. Woodward 

 has supported Perrier's view in a tentative fashion, but the most recent work inclines in 

 favour of the older opinion. In particular. Miss Drummond has confirmed Erlanger's 

 ontogenetic work, and Thiele has described a structure in Trochus cinerarius which 

 resembles the nephridial gland and is here an outgrowth of the left kidney, as that of 

 Monotocards is an outgrowth of their single kidney. The difficvilty remains that it seems 

 almost impossible to derive the Monotocard kidney from an organ specialized in another 

 direction, such as is the case with the left kidney of Trochus and even of Pleurotomaria. 

 We should therefore, apparently, have to derive the Monotocards from forms more 

 primitive than Trochus or Pleurotomaria, in which the left kidney had not yet 

 acquired the special epithelium and the highly peculiar circulatory arrangements of a 

 papillary sac. 



However this may be, it would seem to correspond best with the facts were we to 

 endow the Prostreptoneure with a left kidney, more compact and much smaller than 

 the right, but still retaining ordinary excretory epithelium. The Prostreptoneure 

 possessed a single gonad whose duct led either into the pericardium or, more probably, 

 into the rtno-pericardial canal of the large right kidney, as is the case, at any rate, in 

 Saliotis and Trochus. 



At the end of this account of the hypothetical ancestor of the Prosobrancbs it 

 seems important to urge that no attempt is made to derive tliat group from the 

 Docoglossa, which are certainly highly specialized on lines of their own. It is only 

 supposed that the Docoglossa diverged from a very primitive Prosobranch stock, and 

 thus, though the divergence may be considerable, the gi'oup is an important one in 

 connection with the problems of Gastropod phylogeny. 



The difference betweeu the view here developed and that brought forward by Thiele 

 touches the extent of the external asymmetry of the ancestral form. Tliiele, influenced 

 by the paired ctenidia and the many other admittedly primitive characters of Pleiiro- 

 tomaria and Haliotis, would imagine an ancestor somewhere intermediate between the 

 two, not so high as the former, but not so distinctly flattened as the latter. It possessed 

 a paired arrangement of the shell-muscle, but the spiral was already very distinctly 

 asymmetric. 



Such a view appears to me to connect too closely the development of the torsion and 

 that of the conical spiral hump. In a previous essay (22) I have endeavoured to clear 

 up one or two difficulties in connection with the former process, and to show more 

 especially why the torsion was essentially a forward and upward movement of the 



