OF MASSACHUSETTS. 15 



secretions differ in structure; the inner pearly or nacreous portion 

 being formed of thin layers, the outer of prisms. The valves increase 

 in size as the direct consequence of the increase in size of the soft parts. 



The shell is well adapted to the life of the scallop. Being light in 

 weight it is suitable for movement through the water, while the rounded 

 outline is the form which offers the least resistance for swimming. The 

 opposite ridges and furrows fit tightly together when the valves close. 

 Thus, when the animal moves, streams of water are forced by the aid 

 of the mantle through the small openings below the " ears," or from 

 the ventral edge of the shell. 



The Mantle. The shell of the scallop is lined with a thin ciliated 

 organ called the mantle (Fig. 73, m). The thickened margins of the 

 two mantle lobes are free ventrally but are united dorsally and to a 

 slight extent on the anterior and posterior ends in the region of the 

 " ears." In a scallop of 52 millimeters the mantle lobes are united 

 posteriorly for 13 millimeters and anteriorly for 6 millimeters. This 

 corresponds roughly to the width of the " ears " of the shell, 12 milli- 

 meters and 6 millimeters respectively. The shorter union anteriorly 

 permits- the extrusion of the foot. The free edge of the mantle, often 

 brightly pigmented, possesses tentacles (t) or tactile organs, and bead- 

 like eyes of a bright blue color. These sensory organs are not so 

 numerous or so large near the byssal notch and the corresponding 

 posterior edge. Each lobe of the mantle is attached to the inner surface 

 of the valve about one-half inch from the free edge. The broad face 

 of each lobe rests on the inner surface of the shell except when the 

 animal is disturbed and the mantle withdrawn by the retractor muscles. 

 While resting, the mantle lobes are held slightly apart, the guard tenta- 

 cles forming a lattice work between the perpendicular flaps of the 

 mantle through which the water passes. As previously stated, the 

 mantle also functions in the formation of the shell. 



Tentacles. The sense organs of the mantle are of two kinds, tenta- 

 cles and eyes. There are numerous tentacles near the free edge of the 

 mantle, which vary greatly in size and formation. These tentacles can 

 be divided into two classes, (1) the mantle tentacles, which are situated 

 on the external edge of the lobe in several rows, and (2) the guard 

 tentacles, situated on the edge of the perpendicular flap of the mantle, 

 5 to 6 millimeters from the edge. Each class differs in form and func- 

 tion; the mantle tentacles are larger, capable of greater extension and 

 contraction, and armed with papillary projections, while the guard 

 tentacles are smaller, less extensible and of a bright color. The tentacles 

 have a sensory or tactile function, and when the scallop lies undis- 

 turbed, the mantle tentacles, lengthening out, wave slowly in the currents 

 of water. They can be withdrawn immediately at the passing of a 

 shadow or at any slight disturbance in the water. When contracted 

 each forms a slight conical projection. 



Eyes. Situated between the band of tentacles and the outer edge 



