614 DR. R. V. ERLANGER. 
The history of the development will explain the facts we 
have met with in the anatomy of the forms dealt with in this 
paper. In Fissurella and allied forms, as well asin Patella 
and Tectura, both renal organs have lost their communica- 
tion with the part of the celom represented by the pericar- 
dium. The shape and size of the right kidney, and the way it 
extends between all viscera, pervading the whole body of 
Fissurella and Patella, are explained when we consider it 
as a part of the celom. The fact that the nephridia in Fis- 
surellide and Patellide have lost their communication 
with the pericardium is certainly surprising, especially as no 
other such cases are known in molluscs. In other groups, 
however, we meet with parallel cases—so, for instance, in 
Hirudinea, in which most species have lost the opening of the 
nephridia into the celom. Besides, it must be remembered 
that both Fissurella and Patella, which by reversion to the 
primitive external symmetry have regained or preserved to a 
great extent the original internal symmetry, are in reality 
highly modified forms. This is abundantly shown by the 
story of the development of the shell (loc. cit.). 
I will now attempt to give an idea of the condition of the 
uro-poetic apparatus in the hypothetical ancestral form of 
molluses, and to show how the present condition in actual 
living groups can be easily deduced therefrom (see diagram). 
It is now a view accepted nearly by all morphologists, that - 
the ancestral form of molluscs was of bilaterally symmetrical 
build, and that the anus (4.) and mantle (M.) cavity were 
situated at the aboral end of the animal. Spengel,! Biitschli 
(5), Ray Lankester (21), Lang (17), and others have built up 
their theories on the asymmetry and torsion on this hypo- 
thesis. The existence of the bilateral symmetry further 
implies that the ancestral form possessed paired ctenidia 
(Br.), osphradia, and nephridia (N.). The nephridia were 
tubes with a medium glandular portion having a communica- 
tion (Y.) with the pericardium (Pc.), and opening at the basis 
1 Spengel, J. W., “ Die Geruchsorgane und das Nervensystem der Mol- 
lusken,” ‘ Zeitschr. f. wiss. Zool.,’ 1881, xxxv, pp. 3838—384, Taf. xvii—xix. 
