ANATOMY AND PHYSIOLOGY. 
Moll, 9 
animal only in a straightforward direction. The other system consists of 
true muscular fibres, acting by contractions and only under the simulta- 
neous influence of the nerves ; they are disposed in an oblique direction and 
alone cause the lateral bending in the gait of the animal and the real 
shortening of the foot. In Avion and Limax, the locomotive fibres occupy 
only the middle longitudinal area of the lower surface of the foot; in 
they are intermingled with the others through its whole 
breadth. A small Helix can move along even when burdened with a 
weight ninefold its own. Z. wiss. Zool. xxx. suppl. vol. pp. 166-224, pi. 
viii. ; abstract and some additions in Z. ges. Naturw. (3) hi. pp. 381-^383. 
A. CouTANCE [sMprd] has made various interesting experiments on the 
muscular contraction in Bivalves, chiefly Pecten maximus. A weight of 
about 10,000 grammes, or somewhat more, is needed for opening by force 
a contracted Pecten, the weight of which with the shells is 200, without the 
shells, 85 grammes ; and then the adductor is suddenly rent, not extended. 
The borders of the mantle are the most sensitive part of the animal ; 
they contract when irritated four days after the removal of one valve, 
and also after extirpation of the heart. The heart itself is less sensitive 
than the adductor muscle. A Pecten kept closed for nineteen consecu- 
tive hours, by imposition of a weight of 600 grammes, and then released, 
never opened itself, but proved after four days to be dead in the con- 
tracted state. A heat of 35°-45‘^ 0. annuls the contractility of the 
adductor muscle by galvanic irritation, cold of — 6® 0. for twenty minutes 
renders the animal stiff and insensible ; the sensibility is restored for a 
short time by heat of 35°-40°, but after some hours it ceases totally. 
Injections of toxical substances destroy the muscular sensibility only 
slowly and uncertainly; mechanical irritation has only local effects, 
puncture by a red-hot needle has no more effect than a cold one ; gal- 
vanic irritation exhibits the most sure and extensive effect. The mode 
of contraction, chiefly in the borders of the mantle, is different according 
to the modes of irritation. The adductor muscle is composed of two 
parts ; one, consisting of striated fibres, has its natural volume, and is 
therefore at rest when the valves are a little opened ; the other one being 
nacreous, consisting of smooth fibres, is even extended when the valves 
are closed, and has therefore always a tendency to contraction, and so the 
muscle as a whole is never totally at rest during the life of the animal. 
In the Oyster, the same two parts of the adductor muscle can be dis- 
tinguished, but both consist of smooth fibres, those of the non-nacreous 
part being undulated and transparent. An Oyster of 144 grammes, with- 
out shell 12 grammes, yielded to the closing of its valves only under the 
traction of 10 kilogrammes. In Anomia, both muscles, the external fixed 
to a foreign object and the internal closing the valves, are composed 
of both these elements. In most Dimyaria, for instance Venus and 
Tapes, both adductors are also composed of both elements ; but in Pect- 
unculus one adductor consists only of the nacreous element, in the other 
the transparent much prevails. 
H. V. IiiERiNG also distinguishes the same two parts in the posterior 
adductor muscles of several Bivalves, and in that of Pecten, and calls the 
one of yellowish colour the muscular, the other (of whitish-blue colour) 
1878. [vOL. XV.] B 10 
