741 
1888-89.] Mr D. M‘ Alpine on Bivalve Molluscs. 
ciliated non-rotating embryo of Sepia recalls the cases of the non- 
rotating ciliated detached parts, even although the cilia themselves 
are in active motion. 
After these general references, the rotation of the embryos of 
Mytilus, Unio, and Ostrea will now be considered. The develop- 
ment of Mytilus edulis, L., has been recently and specially studied 
by J ohn Wilson, Demonstrator of Zoology, University of St Andrews, 
and he has found that cilia cover the greater part of the surface of 
the embryo, causing it to rotate actively, but no idea is given of 
rotation.* * * § 
In Anodonta and Uhio there is likewise rotation of the embryo, j 
In Unio litoralis, w r hen the rotation is most active, 7 or 8 revolutions 
are said to be observed per minute,! but this is presumably, as seen 
under the microscope, and therefore of no value without the magni- 
fication. In Anodonta intermedia the rotation is at the rate of 
from 4 to 1 rounds per minute (15 to 79 seconds), § but the same 
remark probably applies. 
Although the development of the oyster has been recently and 
carefully studied, there is no mention of rotation of the embryo 
within the egg, although there is a double oval ring of cilia. 
Probably it does occur, as in the larva of Cardium, which it other- 
wise closely resembles. 
The rotation of the embryo in the ovum of the frog is described 
as being from right to left, at a rate of from 5 to 1 2 minutes per round 
(Pfluger’s Archiv , 1870, Heft 2 and 3). Even at the highest rate, it 
is slow, as compared with some of the detached parts already con- 
sidered, and with the embryo of Unio, for instance. Eor proper 
comparison, however, the relative sizes would require to be taken 
into account. The rate of rotation, so seldom given, is worthy of 
attention, particularly in a case like Mytilus, where the adult still 
retains a power only possessed by the embryos of higher forms. 
The movements of the detached parts of the three chosen forms 
have thus yielded valuable results, and to a certain extent these 
forms are representative for our present purpose. There is the free 
* First Annual Report of Scottish Fisheries Board. 
t Balfour’s Comparative Embryology , vol. i. p. 266. 
+ Owen, Lectures on Invertebrate Animals , p. 526. 
§ Bronn’s Thier-reich. 
VOL. XVI. 21/1/90 3 B 
