584 The American Naturalist. [June, 
the terms themselves, and that, properly speaking, the process found 
in the group is clearly to be regarded as a true alternation of asexual 
generations. 
Body-Cavities of Paludina vivipara.—A short preliminary 
notice is published in the Zool Anzeiger for February 23d, by R. v. 
Erlanger, on the ‘‘ Development of Pa/udina vivipara.” The descrip- 
tion of the origin of the body-cavities is interesting. The gastrula 
arises by invagination. ‘‘ Soon the archenteron pushes out (aus stiilpt 
sich) at the sides and ventrally, so that one sees in side-view of the 
embryo two sacs (Schläuche), one long dorsal, one the archenteron, 
and a shorter ventral one the ccelom sac. Soon the ccelom sac pinches 
off from the archenteron, and surrounds it ventrally and at the sides. 
In course of development the mesoderm cells (which before formed a- 
mass with the cells in close contact) separate more from each other, 
forming a parietal and visceral layer, at the same time growing around 
the archenteron dorsally. Lastly, the mesoderm breaks up into the 
characteristic spindle-cells which run irregularly through the the body- 
cavity. In the meantime the cesophagus arises by invagination of the 
ectoderm, and connects with the archenteron, while the gastrula mouth 
(blastopore, Urmund), as is known, is converted into the anus. At this 
stage the mesoderm collects ventrally in the archenteron, not far from 
the hinder end of the body, in two cell-masses, and in these soon a 
lumen appears. The sacs so formed press together in the ventral 
mid-line until they fuse with one another and fuse into a single mass, 
whose paired origin is for a long time indicated by a middle septum. 
In this way is formed the sac of the pericardium.”’ 
