612 DR. BR. V. ERLANGER. 
duct is formed by a short invagination of the ectoderm. The 
external opening is clearly seen in the veliger stage to the left 
and a little below the anus. According to Signor Mazzarelli 
there is even evidence to prove that a similar but rudimentary 
renal organ was, before the torsion, situated to the left of the 
rectum. 
In Pulmonata and Tectibranchiata the torsion (leaving 
out of account the torsion of the visceral hump, which is an 
independent process [Biitschli] from that of the general tor- 
sion) is already less marked than in Prosobranchia. The 
original twisted condition is still more reduced in Pteropoda 
and Nudibranchiata, where a kind of untwisting has very 
probably taken place. This explains why the actual left 
kidney (the right one before the torsion) apparently does not 
occupy the same position as in Gasteropods, though in reality 
its position is homologous, provided we suppose that a 
secondary untwisting has taken place to a greater or lesser 
degree. 
In all other groups of Mollusca, that is to say, in Placo- 
phora, in Scaphopoda, in Lamellibranchiata, and 
Cephalopoda, the renal organ is paired. 
I havenot yet mentioned the Aplacophora or Neomenia, 
as Pruvot (25) has recently thrown considerable doubt on 
Hubrecht’s (15) views on the morphology of the uro-poetic 
apparatus in this interesting group. Pruvot considers the 
ducts leading from the pericardium (Hubrecht) as nothing but 
oviducts, the pericardium itself as a mere egg-pouch. The 
unpaired glandular part, considered by Hubrecht as the secret- 
ing part of the nephridia, is called a shell-gland by the French 
anatomist. Pruvot describes as an excretory gland a new 
organ which he calls the cloacal ridge (bourrelet cloacal). 
I cannot attempt to explain away Pruvot’s statements con- 
cerning the secreting part of the kidney, not having studied 
myself the anatomy of Neomenia. However, I think that this 
can be easily done for the pericardium and ducts of the 
nephridia. 
Pruvot, after having in a preliminary account of his investi- 
