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Nachdruck verboten. 



Striped Muscle in the Mantle of Lamellibranchs. 



By W. J. Dakin, M. Sc, 

 1851 Exhibitioner, Zoology Depart. Liverpool University. 



With 5 Figures. 



The following notes have been drawn up on some observations 

 made during a sojourn at the Zoological Station of Naples, whilst 

 working on the nervous system and sense organs of Pecten. 



The study of striped muscle in the group of the Mollusca has been 

 advanced during the last thirty years by a considerable number of 

 workers, who have in particular called attention to the double oblique 

 striping seen in Cephalopods and also in the adductor muscle and 

 cardiac muscles of certain Lamellibranchs (see Bibliogr.). Attention 

 has been drawn also to the presence of transverse striation in that 

 part of the adductor muscle of Pecten which performs the rapid move- 

 ments causing the clapping of the shell and hence the swimming (2). 

 I have seen however no reference to striped muscle in the mantle edge 

 of Lamellibranchs. This occurs in Pecten (two species only were ex- 

 amined, P. jacobaeus and P. opercularis) and the striation is not 

 oblique like the cardiac and adductor muscles of most Lamellibranchs, 

 but apparently transverse like the adductor muscle of Pecten, though 

 perhaps not quite so regular. The mantle edge of Pecten is very 

 highly developed. It is thick, very muscular, and in addition to 

 bearing highly organised sense organs takes an equal share with the 

 adductor muscle in producing swimming movements. The edge can 

 be divided roughly into three parts, seen in the transverse section 

 Fig. 1, viz. a shell fold (Sh.f.) separated by the periostracal groove 

 {Per.G.) from the ophthalmic fold (Oph.f.) bearing the eyes, and the 

 large flap, the velum. The velum is practically entirely made up of 

 muscles and it is this part of the mantle that functions in swimming. 



At the moment that the two valves of the shell are brought 

 together by the contraction of the adductor muscle, the velum is 

 raised quickly so that it takes up a position at right angles to the 

 mantle surface. The two vela then touch and by means of their 



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