226 G1LMAN A. DREW. 



accordingly borne along the margins of the lobes of the mantle. 

 It is not entirely certain what relationship Pecten bears to the 

 usual form of lamellibranch as regards positions of parts. In 

 lamellibranchs that are supplied with two practically equal 

 adductor muscles, a line connecting the two adductors runs 

 nearly lengthwise of the animal. In such a case the hinge line 

 is more or less dorsal, one end is anterior, and the other pos- 

 terior. When one of the muscles disappears, as is the case with 

 Pcctcn, one of the landmarks disappears and it becomes more 

 difficult to locate the direction of parts. Inasmuch as the hinge 

 line is usually dorsal, it is very natural to look at the hinge line 

 of this form as dorsal, and for matters of description it is con- 

 venient to so consider it. If, however, the position that the 

 anterior adductor would have occupied, had it been retained, be 

 considered, the position of the mouth, foot and heart indicate 

 that it would have to be placed much nearer the hinge line than 

 the present position of the posterior adductor muscle, the muscle 

 that is retained. If this is the case, it becomes evident that the 

 loss of the anterior adductor muscle has been accompanied by a 

 general reduction of the anterior part of the body, so a large part 

 of the body of Pecten is to be considered morphologically pos- 

 terior. This supposition seems to be borne out by the nervous 

 system, and the vascular system of the mantle, as well as by the 

 extent and position of organs. In most forms the margin of 

 each lobe of the mantle is supplied with a posterior and an ante- 

 rior pallial nerve of approximately equal size. These nerves 

 supply the muscles and sense organs of the margins, and, in 

 many forms at least, unite with each other so they form a con- 

 tinuous connective between the cerebral and the visceral ganglia. 

 In Pcctcn, not only is this the case, but the nerve in the margin 

 of the mantle is joined at intervals for nearly its whole length 

 by nerves from the visceral ganglion (Fig. 6). On the other 

 hand, it is joined only in the region of the anterior ear by 

 nerves from the cerebral ganglion. The visceral ganglia are the 

 important ganglia of the animal, and both the cerebral and pedal 

 ganglia are greatly reduced. 



The blood is supplied to the mantle very largely by the pos- 

 terior pallial arteries (Fig. 5). The anterior pallia! arteries are 



