NOTES AND MEMORANDA. 
Development of Planorbis. — Dr. Carl Rabl, of Vienna, has 
published in Gegenbaur’s ^Morphol. Jahrbuch,’ vol. v, a 
very carefully illustrated account of the development of the 
fresh-water snail Planorbis. In essentials the history does 
not differ from that of Limnseus, on which subject Dr. Rabl 
some time since published a less complete and less accurate 
memoir (‘ Jenaische Zeitschr.,* 1875). In the present 
memoir Dr. Rabl abandons his erroneous interpretation of 
the shell-gland and recognises the correctness as well as the 
priority of ray descriptions of that organ in the Mollusca 
generally and of the velum of Pulmonata. Dr. Rabl has, 
however, failed to trace the derivation of the hinder part of 
the embryonic intestine from a group of cells adhering to 
the margin of the blastopore (constituting a “ pedicle of in- 
vagination ” as in the Lamellibranchs Pisidium and Ano- 
don) ; on the contrary, he considers this portion of the enteric 
tract to be a caecal outgrowth of the invaginated endodermal 
sac, which outgrowth pushes forward so as to touch the 
body-wall. The endodermal sac (archenteron) is only in 
contact with the blastoporal margin (according to Dr. Rabl) 
at the anterior or oral end of that orifice. Satisfactory evi- 
dence of this is not adduced. Drawings of a very beautiful 
series of sections are given in two plates as well as numer- 
ous transparent views of young embryos. The drawings of 
sections are perfectly accurate, as I am able to state from 
the examination of a number kindly sent to me by Dr. 
Rabl. The sections are masterpieces of embryological mani- 
pulative skill, but they do not tend to substantiate the view 
with regard to the origin of the hinder part of the embry- 
onic intestine which Dr. Rabl maintains. They are equally 
favorable to the view to which I was led by observations 
recorded in this Journal in 1874. Dr. Rabl takes so 
strongly adverse a position in reference to the connection of 
the hind-intestine with the blastopore, that he emphatically 
denies the accuracy of my observations on the connec- 
tion of the blastopore and anus in another Gasteropod, viz. 
Paludiiia. 
