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POPULAR SCIENCE REVIEW. 
account of the Vertebrata, and treats of the Mollusca and Molluscoida. 
The publication of the parts of this work has certainly been irregular, the 
second volume having appeared in 1863, and the first part of the first volume 
in 1868. The reason of this is now fully explained in the preface. Dr. 
Peters, of Berlin, who had originally undertaken the Vertebrata, was unfor- 
tunately forced by pressure of other occupations first to delay and then to 
relinquish the execution of the task ; and thus, with the exception of the 
Arthropoda, which were treated of by Herr Gerstaecker, the whole weight of 
the work eventually fell into the able hands of Professor Carus. 
The Development of the Oyster is thus fully given by Mr. Packard (in the 
“ American Naturalist,” May 1875). The course of development is thus : 
After the segmentation of the yolk (morula stage), the embryo divides into 
a clear peripheral layer (ectoderm), and an opaque inner layer containing the 
yolk and representing the inner germinal layer (endoderm). A few filaments 
or large cilia arise on what is to form the velum or the future head. The shell 
then begins to appear at what is destined to be the posterior end of the 
germ, and before the digestive cavity arises. At this stage the two-layered 
germ is said by Salensky to represent the planula of the sponge. The diges- 
tive cavity is next formed (gastrula stage), and the anus appears just behind 
the mouth, the alimentary canal being bent at right angles. Meanwhile 
the shell has grown enough to cover half the embryo, which is now in the 
“veliger” stage, the “ velum ” being composed of two ciliated lobes in 
front of the mouth-opening, and comparable with that of the gastropod 
larvae. The young oyster, as figured by Salensky, is directly comparable 
with the veliger of the cardium. We have, then, three stages of growth 
in the oyster: (1) the morula, (2) the gastrula (with the digestive cavity as 
yet undeveloped), and (3) the veliger with an alimentary canal and a head 
and hind body (cephalula). This is an epitome of the mode of development 
of most of the lamellibranchiate molluscs whose embryology is known. 
Soon the shell covers the entire larva, only the ciliated velum projecting 
out of an anterior end from between the shells. In this stage the larval 
oyster leaves the mother and swims around in the water, the cilia of the 
velum keeping up a lively rotary motion. In this state Lacaze-Duthiers 
observed it for forty-three days, without any striking change in form, except 
that the velum increased in size, and the auditory vesicle appeared, contain- 
ing several otoliths, which kept up a rapid motion. But still the gills and 
heart were wanting. Of its further history we know but little, except that 
it becomes fastened to some rock and is incapable of motion. The oyster is 
said, by the appearance of its shell, to be three years in attaining its full 
growth j but this statement needs confirmation. 
