42 THE SCALLOP FISHERY 



During the dissoeonch or attachment stage, the mantle first takes on 

 characteristics which differentiate it from the early stages of other 

 forms. The edges become more folded and knob-like projections grad- 

 ually form at definite places on the border, some to form tentacles, 

 others the eyes of the scallop. ( The development of the eyes and tenta- 

 cles will be considered separately under " Sensory Organs.") The 

 retractor muscles become stronger, and the mantle is now capable of 

 greater extension and contraction, withdrawing at points where irritated. 

 As the animal grows larger the number of retractor muscles of the 

 mantle increase and are attached in a widening semi-circle far down the 

 interior of the shell, so that only the outer portion of the mantle hangs 

 free. 



Another important functional change takes place when the so-called 

 flap of the mantle is formed. This is a thin outgrowth in a perpendicu- 

 lar direction along the entire edge of the mantle, except just beneath 

 the " ears " near the siphonal openings. The flap, when first formed 

 during phase 6, is entirely plain, but soon is ornamented with a row of 

 small tentacles called by Jackson (4) "guard tentacles." With the 

 formation of the guard flap the animal has become a specialized scallop, 

 differing from other lamellibranchs. The valves are now held apart, 

 when resting, in such a way that the opposite flaps almost close the 

 intervening space. Water can be taken in and shot out of the shell, 

 giving the scallop the power of swimming. 



Closely allied in function with the guard flap is the formation, during 

 phase 5, of what is known as the pseudo-siphon, which arises as a 

 transparent conical projection from the median posterior border of the 

 mantle. This organ is formed by the concrescence of the mantle edges, 

 and is not a true siphon, as is found in the clam and quahaug. Func- 

 tionally this pseudo-siphon acts as an excurrent canal to eject water 

 from the shell. Although it is not used for the purpose of swimming, 

 as is the case with the same region in the adult scallop, it assists the 

 animal at this period of life in crawling, as simultaneously with the 

 contraction of the foot a stream of water is ejected from the pseudo- 

 siphon. After each flow of water the siphon is retracted again, to be 

 extended when the next stream is forthcoming. The pseudo-siphon 

 disappears before the scallop reaches adult size, and is evidently only 

 functional during the crawling period. 



The mantle, particularly the edge, is beautifully hued with many 

 colors. The mantle of the scallop at first is a transparent white, which 

 gradually takes on the colors of the adult mantle. The intensity of the 

 color varies greatly in the different scallops and is as unexplainable as 

 the variety of colors in the shell. 



The Sense Organs. --The scallop has a well-developed sensory sys- 

 tem of specialized parts, each of which contributes to the maintenance 

 of life and to the protection to the animal. 



(a) Tentacles. The tentacles in the adult scallop line the border of 



