OF MASSACHUSETTS. 45 



(c) The Otocyst. — The otocyst, or organ of equilibrium, is situated 

 in the foot in the young animal. It is first seen in the scallop of the 

 prodissoeoneh "stage as two vestibules of small size, one on each side 

 of the foot. Inside the circumference is a clear fluid in which several 

 small granules are constantly revolving (Fig. 20), evidently due to 

 ciliary action. These remain prominent in the foot as long as that 

 organ is relatively the largest part of the body, but are gradually lost 

 sight of in the visceral mass of the adult scallop. 



The Gills. — The gills form during the transition period from the 

 early veliger to the prodissoeoneh stage, when they are observed as 

 simple primitive folds lined with vibrating cilia. At the beginning of 

 the dissoeonch stage the gill is a bar folded into four simple filamentous 

 processes, covered on the outer edge of the folds by rapidly stroking 

 cilia (Figs. 18 and 20). Later stages show that the bar filaments are 

 added ventro-posteriorly, first appearing as bud-like processes. The 

 gills then consist of simple bar filaments so arranged with the longer 

 ones dorsal that the whole gill has a semi leaf-like appearance. At the 

 end of the dissoeonch period these filaments number between 20 and 25, 

 while the more mature dorsal bars became enlarged at the free end, 

 due to their turning back upon themselves. At this time there are two 

 gills, one on each side of the body, which are the inner gills of the adult. 

 This later growth marks the beginning of the inner lamella although the 

 filaments are still separate. The scallop is about 1 millimeter (about 

 ^^5 of an inch) in size at this time. 



The next change is the formation of two outer gills, which mark the 

 characteristic structure of the adult. Just previous to the appearance 

 of the outer gills the animal has two inner gills of about seventeen 

 filaments. Small •' buds " arise on the upper edge at the posterior end 

 of the gill, and increase rapidly in size and number. It is curious that 

 the development of the outer gills starts at the posterior instead of the 

 anterior end, exactly reverse to the formation of the inner gill. In a 

 1%-millimeter scallop sixteen filaments were counted on the outer gill, 

 in a 1.8-millimeter scallop twenty-eight, and by the time the animal had 

 reached the size of 3 millimeters the gills had the same appearance as 

 in the adult. The inner gills are reflected inward, the outer gills fold 

 outward to form the second lamella. 



The later changes are more complicated and not so conspicuous. The 

 filaments appear to become united, but on close examination this union 

 is found to be due to the interlocking of ciliated discs on the posterior 

 and anterior sides of each filament, giving the appearance of interfila- 

 mental cross bars. The filaments are joined in groups or bands of seven 

 or eight. A 4-millimeter scallop has about ten bands, a 5-milHmeter 

 specimen twenty-five. The lamelte are also attached at intervals by a 

 fine septum. 



The gills are at all times very sensitive. When touched with a pencil 



