ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
583 
nerve externally, while, ventrally, there is a strong nerve-cord which is 
directed towards the pedal ganglion, which, about half-way on its course, 
becomes united with the cerebro-pedal connective. 
In Solenomya there is a similar arrangement, with the sole difference 
that the nerve -fibres which go from the ganglion at the origin of the 
visceral commissure as far as the pedal centre join those of the cerebro- 
pedal connective on their exit from the ganglion ; in this way the common 
trunk which they form arises from the junction of the cerebral ganglion 
with that which is connected with it posteriorly. 
If we compare the arrangement observed in Nucula and Solenomya 
with that which obtains in Gastropods and Dentalium , we see that the 
ganglion from which arise the anterior pallial nerve, the visceral com- 
missure, and the fibres which pass to the pedal centre, is the pleural 
ganglion, while the fibres which join this last centre to the pedal 
ganglion of Nucula and Solenomya form the pleuro-pedal connective 
which was believed to be wanting in the Lamellibranchiata. In such as 
are more specialized than these two Protobranchs the pleural and cerebral 
ganglia are fused into a single ganglionic mass (which is always called 
the cerebral), as may be seen when sections of the mass are made, and 
the two connectives — the cerebro-pedal and pleuro-pedal — are united for 
their whole length. 
Progression and Rotation of Bivalve Molluscs and of Detached 
Ciliated Portions.* — Mr. D. M‘ Alpine has continued | his observations 
and experiments on this subject, and now gives an account of what he 
has observed in the freshwater mussel ( Unio ), in which the general results 
are much the same as with Mytilus, and in the Oyster. 
When the movements of these three forms are compared in their 
natural condition, it will be found that Unio has the greatest activity, and 
Ostrea, as far as known, the least ; but if the progressive and rotatory move- 
ments due to cilia are in question, then Mytilus undoubtedly takes the lead. 
Each of these three forms has a distinct and specially active part, sugges- 
tive of underlying differences ; in Mytilus it is the gill, in Unio the ventral 
margin of the foot, and in Ostrea the labial palp. The cilia are supposed 
to continue their work without any rest, but it may be imagined, in a 
structure like the gill, with its innumerable cilia, that they rest in relays 
without interfering much, if at all, with the general effect. 
In the course of his investigations the author noticed an important 
distinction between the action of the cilia and the movement of the cilia- 
bearing mass. The movement of the mass might cease and yet the cilia 
themselves, when examined under the Microscope, would be in active 
motion. The cilia in themselves are, therefore, not the cause of move- 
ment ; there has to be co-operation or co-ordination of some sort before 
the ciliary motion can give rise to movement of the part bearing the 
cilia. Ciliary motion which causes currents in streams must, therefore, 
be distinguished from ciliary motive power. 
Organ of Boj anus in Anodonta cygnea.J — Dr. W. M. Raukin gives 
a very full account of his observations on the organ of Bojanus in the 
* Proc. Roy. Soc. Edinb., xvi. (1888-9) pp. 725-43 (2 pis.). 
f See this Journal, 1889, p. 739. 
X Jenaische Zeitsclir. f. Naturwiss., xxiv. (1890) pp. 227-67 (2 pis.). 
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