53$ 



PRINCIPLES OF GENERAL PHYSIOLOGY 



shorten actively and, indeed, if tested by pressing a hard ohject upon it, it is found 

 to be quite soft when relaxed and as hard as cartilage when in the contracted state. 

 If Fig. 171 be examined, it will be seen that the adductor muscle consists of 

 t\\o distinct parts, a larger round part, which has a clear appearance and consist v 

 ot' cross striated fibres, and a smaller more opaque part, of smooth fibres. In order 

 to distinguish them, we may call, with von Uexkiill, the former the "motor" 



Al.c.5 



fc Go.o. 

 Ko.rp. 



*"|f" " 



- Al.c.4 



P.M.r. 



Br.fl. 

 Br.a. 



; \\ 



Mn. 



FIG. 171. ANATOMY OF PECTEN. RIGHT VALVE AND RIGHT LOBE OF MANTLE REMOVED. 



Aur., Auricle. 



Ven., Ventricle. 



Per., Pericardium. 



A.v., Non-striated part of adductor muscle. 



A.g., Striated part of adductor muscle. 



Br.a. and d., Lamellae of gills. 



Tn., Tentacles. 



Tn.v., Velar tentai-les. 

 ., Eye. 

 Mn., Mantle. 

 I'., Velum. 

 P., Foot 



Lff.p., Litratnent ]>it. 

 .1 /.c. 1 to 5, Parts of alimentary canal. 



(Reproduced by permission of the Liverpool Marine Biology Committee. 

 For the meaning of the remaining letters, see the original monograph 

 by W. J. Dakin, 1909, pi. 2, Fig. 1.) 



muscle and the latter the "catch" muscle. The reason is that, if the "catch 

 muscle be removed, the motor muscle can be excited to contraction and keeps the 

 shells closed as long as the stimulation lasts; but, as soon as this ceases, the elastic 

 hinge causes them to open again. If the " catch " muscle be cut through while the 

 shell is closed, the other muscle is unable to keep it closed ; whereas, if the motor 

 muscle be divided, the shell remains closed. Thus the large motor muscle seems 

 to bring the shells together quickly and for a moment ; the smaller catch muscle 

 then holds them fast in the position to which they have been brought by the 



