THE CIRCULATORY AND NERVOUS SYSTEMS OF 

 THE GIANT SCALLOP (PECTEN TENUICOSTATUS, 

 MIGHELS), WITH REMARKS ON THE POSSIBLE 

 ANCESTRY OF THE LAMELLIBRANCHIATA, AND 

 ON A METHOD FOR MAKING SERIES OF ANA- 

 TOMICAL DRAWINGS. 



OILMAN A. DREW. 



In considering any system of organs it is essential that we 

 should bear in mind the modifications of the possessor of the 

 organs, that adapt it to its particular life. 



Pecten is one of the ablest swimmers among lamellibranchs. 

 The whole structure of the animal is modified for this purpose. 

 The valves have become rounded in outline, flattened, and com- 

 paratively light. The anterior adductor muscle has been lost, 

 and the posterior adductor muscle, which is very powerful, is 

 situated near the middle of the body. The cartilage has become 

 well developed, so the shell may be opened quickly when the 

 muscle relaxes, and the hinge line is straight, so there may be no 

 unnecessary strains in opening and in closing the shell. Each 

 gill is attached by one lamella only, so water in the temporary 

 cloacal chamber may be thrown out without injuring the gills, 

 and the gills and margins of the mantle are provided with 

 muscles to withdraw them from the margins of the shell when 

 the shell is closed. Furthermore the margins of the mantle are 

 provided with infolded ridges and with circular muscles so it is 

 possible to direct the current of water which issues from the shell 

 in the required direction. 



To fit the animal to a life of such activity, and to enable it to 

 live in the comparatively exposed positions that it inhabits, an 

 abundance of sense organs, tactile and probably visual, have 

 been developed. These are placed in the most exposed posi- 

 tions, where they may give warning to their possessor, and are 



1 Free use has been made of both descriptions and figures published as No. 6 of 

 the University of Maine Studies, under the title of "The Habits, Anatomy, and 

 Embryology of the Giant Scallop (Pecten tenuicostatus, Mighels)." 



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