96 CEYLON PEARL OYSTER REPORT. 



areas of the Cheval Paar. They were pinkish red to the naked eye, and of com- 

 paratively large si/.e, quite ] inch (6 millims.) when extended. From its distinctive 

 habitat, we may appropriately apply the name margaritiferce to this species. 



The body is composed of two distinct regions, an anterior neck-like portion, slender 

 and cylindrical, bearing oral sucker and mouth at the free extremity, and a posterior 

 stout region which spreads laterally, on the ventral aspect, into a broad, oblong, 

 pedal disc, armed with rows of numerous sucker-pits and short, digitate tube-feet 

 (Plate 111., figs. 49 and 50, and Plate IV., figs. Gl and 66). The dorsum is minutely 

 wrinkled or annulated transversely (Plate IV., fig. 62). 



The oral suctorial apparatus (o.s.) is not of the definite rosette form typical of 

 Distomum; it appears as a transverse slit bounded by thin mobile lips. The lips 

 divaricate when about to make adhesion, as in the manner characteristic of an ordinary 

 lipped sucker. 



Careful examination during life showed the pedal disc (p.d.) to possess a 

 wonderfully complex structure. The surface is excavated into numerous cup-shaped 

 hemispherical pits, associated with which are numbers of small tube feet of remarkable 

 characteristics. Both series are arranged with perfect regularity. The shallow pits 

 or suckers (s.p.) are disposed in four longitudinal rows, the individual pits of one row 

 alternating with those of the adjoining, an economy of space which permits the 

 accommodation of the largest possible number of pits (Plate IV., figs. 60 and 61). 

 The mouth of each sucker-pit is simple, bordered by a membranous edge containing 

 muscular elements. When the animal is detached from its hold, the apertures are 

 frequently seen to close by an approximation of the muscular margin. In most cases 

 the edges meet in a tri-radiate manner; in others the lips close upon a single slit, the 

 axis of this being at right angle-! to the adjacent margin of the pedal disc (Plate IV., 

 fig. 62). 



The tube-feet project from the angles between the sucker-pits, forming therefore 

 three double ranks of feet disposed in zig-zag pattern (Plate III., fig. 52, Plate IV., 

 figs. 61, 60, and 68). There are none along the margin of the pedal disc. They are 

 hollow, thin-walled, and tubular, capable of great extension and of complete retraction 

 by inversion, in manner similar to the eversion and retraction of the proboscides of 

 Tetrarhynchus. They are hollow erectile organs of the simplest structure, possessing 

 the power of extension in an extraordinary degree. When fully extended, they 

 assume the form of slender cylinders tapering very gradually to an acuminate apex 

 (Plate 111., fig. 52, Plate IV, fig. 68). Partially retracted, they exhibit a closely 

 annulated or wrinkled appearance, reminding one of the annulation of an earthworm, 

 the anterior extremity of which they greatly resemble. When drawn in more fully, 

 they show as low truncate pillars or stumps. 



The alimentary canal is median and unbranched, ending blindly near the posterior 

 end of the body. The mouth, situated at the base of the oral sucker-slit, leads into 

 a short narrow buccal canal opening into the strongly muscular pharynx (j'li-) oblong 



