20 



dej^ressioii of the mantle (tip-. 1, Jaj. P.). Into this 

 depression the ligament dips, h' ing- transversely across it. 

 There are no fusions of the mantle edge to form separate 

 inhalent and exhalent apertures, and consequently there 

 are no siphons. The inhalent and exhalent currents are, 

 howcA^er, confined to special regions, so that physiologi- 

 cally the fusions are not needed for the sej)aration of the 

 currents. Ba' scattering some carmine into water in 

 which a Pecten is living, the particles of colour can be 

 seen entering all round the shell between the two folds of 

 the mantle, except for an area extending from the 

 posterior end of the hinge line for a short distance 

 forward. Here there is a very definite exhalent current 

 sometimes accelerated by the animal closing the shell 

 suddenly and forcing the water out at this point only, to 

 eject the faeces. 



The free margin of the mantle lobes is much 

 thickened and presents three typical folds (fig. 4). The 

 outer one, the shell fold (fig. 4. Sh. F .), is small and bears 

 long tentacles. The median one, the Ophthalmic fold 

 (fig. 4, Op. F.), is not so distinct and also bears tentacles 

 and the eyes which form conspicuous objects when the 

 animal is alive. The most internal fold is much the 

 largest and is turned inwards to form a flap, known as the 

 "velum" (figs. 1 and 4, V .). It is iisually pigmented 

 either continuously or at regular intervals. List (6) has 

 shown that the storage of pigment in the mantle cells is 

 directly influenced by light, and that removal of a piece 

 of the shell causes a deepening in colour of the tissue 

 exposed, due to formation of pigment. This curtain-like 

 velum becomes reduced in size as it approaches the base of 

 the angle forming the ears, and it is this inner portion of 

 the mantle on both sides that fuses as mentioned above. 

 The outer folds remain fre*^ with their eves and tentacles, 



