16 THE SCALLOP FISHERY 



of the mantle are many well-developed eyes, brilliantly huecl with 

 blue and brown pigment, which help to make the scallop attractive to 

 summer colonists. When sectioned, these eyes are found to be spe- 

 cialized organs comparable to the eyes in the higher animals. They 

 vary in size and number, the larger ones usually occupying the grooves 

 of the shell, although in the adult definite arrangement appears to be 

 lacking. To what degree the eyes react to light and to external stimuli 

 is problematic, and offers a field for research. They appear to be 

 keenly sensitive to any change in light and shade, possibly observing 

 the approach of an enemy by its shadow or movement in the water. 



Muscles. Unlike the soft clam (Mya) and the quahaug (Venus), 

 which have two adductor muscles for closing the shell, the scallop has 

 only a single adductor muscle (the so-called eye in the fisherman's 

 vernacular), which is situated posterior to the center of the shell, form- 

 ing a conspicuous part of the internal anatomy. The muscle is made up 

 of two parts, a large anterior section and a small posterior division. As 

 is shown in the development of the young scallop, this muscle is the 

 posterior, the anterior adductor disappearing during the early shell 

 stage. The muscle is the edible part of the scallop, and its shape is 

 maintained when served on the table as " fried scallops." When the 

 muscle is cut the valves immediately gape open, being forced apart by 

 the V-shaped cartilaginous elastic pad in the middle of the hinge. The 

 other important muscles are the retractor muscles of the foot, the gills 

 and the mantle. 



Gills. -- There are four gills (Fig. 74, g) in the scallop, a pair on 

 each side of the generative mass and ventral to the adductor muscle. 

 Each pair has a free end posteriorly and extends in a curved line 

 nearly around the posterior adductor muscle. The gills are attached 

 dorsally near the adductor, the inner and outer gills having a common 

 membrane. Each gill is made up of two lamellee of radiating filaments. 

 Fine markings cross the filaments at right angles, thus giving each 

 lamella a delicate lace-like appearance. 



When a few grains of carmine powder are sprinkled on the gills the 

 small grains pass to the base of the gills and then move toward the 

 anterior end in a definite channel. This movement is due to the cilia 

 between the filaments, which cause the grains to pass toward the mouth. 

 The work of the gills is not only to strain out the food but to aid in 

 respiration. The impure blood flows into the capillary spaces of the 

 gills, where the delicate membranes are bathed by the inflowing water, 

 and, having acquired a new supply of oxygen, passes back to the heart. 

 The area is increased by the folding of the lamella. If stimulated, the- 

 gills contract immediately, showing that they possess a nervous mech- 

 anism. 



Palps. Just dorsal to the gills on the borders of the mouth are 

 two pairs of delicate filaments similar in structure to the gills. These 



