MOLLUSCA — PELECYPODS 



231 



The animal swims rapidly by flapping its valves together, 

 thus ejecting water from the mantle chamber. When swimming, 

 its hinge edge is usually directed backward and the propelling 

 jet of water is forced from near the right and left ear alternately, 

 thus acting like an oar in producing a zigzag coarse (Fig. 99). 



It lives usually only a little over a year, spawning but once ; 

 very rarely does it reach a second spawning season. Its enemies 

 are not many nor very destructive. 



Fig. 



In the development of Pecten from the earliest shelled condi- 

 tion to that of the adult, remarkable changes occur. The very 

 young shell (the prodissoconch), about .2 mm. in diameter, 

 has a smooth surface, a very convex 

 umbo, a curved hinge line with ten 

 pairs of teeth fitting into correspond- 

 ing sockets, with probably two adduc- 

 tor muscles and a non-prismatic shell 

 structure. Immediately succeeding 

 this stage and continuing until the 

 shell reaches a diameter of about i 

 mm. is the dissoconch (Fig. 100, d.). 

 It is characterized by a byssus, a byssal 

 notch, formation of ears, a straight 

 hinge line, a large active foot, pris- 

 matic structure of shell in at least the 

 right valve, and a loss of the anterior 

 adductor muscle. Later, at a diam- 

 eter of about 1.25 mm. (the plicated 

 stage) the number of plications nor- 

 mal to the adult appear all at once. 

 In the adult the foot has almost 

 entirely disappeared, and the shell 

 structure is non-prismatic. The pro- 

 dissoconch has usually been com- 

 pletely eroded in the adult shell. 



Upon comparing the above changes 

 with fossil shells it is seen that the prodissoconch is exceedingly 

 similar to the Ordovician Nucula, the dissoconch to the Silurian 

 Rhombopteria (Fig. 100, R), allied to Pteria, the plicated stage 

 to the Silurian Aviculopecten, the adult Pecten being known 



100. — Plicated stage of a 

 young scallop, Pecten gibbtis 

 borealis Say ; outer surface of 

 right or lower valve; the left 

 valve shows slightly to the 

 right, b.g., b3'ssal groove ; b.n., 

 byssal notch; d., dissoconch; 

 er., ear; pd., prodissoconch or 

 earliest shell ; pi., plications ; 

 R., Rhombopteria stage (the 

 Aviculopecten stage is reached 

 when the plicated stage is 

 about five times longer than 

 represented here) ; te., teeth. 

 Size 1.4 mm. (Redrawn from 

 Belding.) 



