46 JOURNAL OF CONCHOLOGY. 



PRELIMINARY NOTES ON THE 



PHENOMENA OF MUSCLE-CONTRACTION 



IN THE MOLLUSCA. 



By Dr. J. W. WILLIAMS, M.A. 



(Read before the Conchological Society, May 2nd, 1888, and recommended for publication 

 by the Rev. A. H. Cooke, acting as referee). 



The phenomena of muscle-contraction resolve themselves into 

 two great and somewhat distinct primary groups, one of which— 

 CILIARY ACTION— is more simple in its character than the other 

 — MUSCULAR CONTRACTION PROPER. In this paper we shall 

 take, for the sake of more clearness in detail, each of these 

 divisions separately. 



And, first, with regard to ciliary action. Cilia are found 

 in the intestinal canal of most molluscs, and on the gill-filaments 

 of the lamellibranchs where they are more strongly developed 

 at certain spots termed "ciliated junctions." They are fila- 

 mentous prolongations of the protoplasm of columnar, cubical, 

 or sphseroidal cells, the free borders of which are bright and 

 seem to be made up of juxtaposed knobs from whence fine 

 varicose filaments extend into the substance of the cells forming 

 the rootlets of the cilia. From each of these knobs a somewhat 

 long cilium — long when compared with a cilium from the 

 mucous lining of the mammalian trachea — extends. Their 

 function is to execute rapid lash-like movements [motus undu- 

 latus of Valentin) in order to drive the water in a given direction 

 over the gills for respiratory purposes, or, else, to help the move- 

 ment of the fluid ingesta along the intestinal tract onward to the 

 anus. They do not seem to be under the control of the nervous 

 or circulatory systems for the lashing of the cilia obtain when 

 the cells to which they are attached are bodily removed or even 

 when they are isolated from one another, but their motion 



J.C., vi., Jan., 1889. 



