THE PARASITES OF THE PEARL OYSTER. 81 



are highly developed, and consist of two series, one running longitudinally, the other 

 in a circular manner. 



The cuticle of the head and its groove is smooth ; that of the sheath is ornamented 

 with a multitude of closely set minute shagreendike points, resembling in shape 

 denticles from the skin of Scyllium ; the central point having a tiny basal projection 

 on either side (Plate I., figs. 10 and 11, Plate II., fig. 18). 



The hinder portion of this larva is the morphological equivalent of the " bladder " 

 of the Cysticercus stage of Tarda, differing however in that the contents, in lieu of 

 being fluid, are composed of a loose connective-tissue parenchyma, in which are 

 ilistributed the rounded retractile bodies already referred to. These effervesce upon the 

 application of acid, and by this test, taken together with the similarity in appearance, 

 they are shown to be calcareous corpuscles identical with those so widely met with 

 among the Cestoda. 



Ensheathing the body parenchyma are two muscular layers of similar arrangement 

 to those of the head, but of extremely feeble development. These in turn are 

 overlaid by a cuticle of peculiar appearance. In the parenchyma ramify the tubules 

 of the vascular system. 



The connective-tissue framework of the cortical region of the parenchyma is rather 

 denser than it is in the centre. In the former, accumulating at the periphery, lie the 

 majority of the calcareous corpuscles. Of these, while some are nearly spherical, the 

 greater number are slightly reniform, or else obscurely compound, double, or even 

 roughly trifoliate, shallow depressions marking some out into two or even three 

 distinct areas (Plate II., figs. 1G and 17). With pressure, they can be broken into 

 irregular sharp-edged fragments, just as a glass ball shatters under a blow. 



The traces of a distinct and fine capsule around many of the corpuscles, which can 

 be detected in the living tissue, is probably the remains of the cells in which the 

 concretions arise (Plate II., fig. 17). 



A closely woven network of anastomosing tubules of exceeding delicacy, suffused 

 with the palest of pink tints, is also to be seen in the cortical parenchyma. Such 

 represents a generalised and rudimentary excretory system which becomes specialized 

 in succeeding stages (Plate I., fig. 1:2). 



In the peripheral region just under the cuticle are occasionally to be made out 

 large clear-walled cells filled with ' colourless transparent globules ; probably these 

 are the " bladder-cells" which ooze through the cuticular tubules under pressure. 



When a living larva is examined microscopically, the body behind the denticulated 

 hinder portion of the head-sheath shows a distinct appearance of being clothed in 

 a densely ciliated envelope. So distinct is the appearance, that time after time 

 references to the " ciliated surface " of these larva? appear in the earlier notes made 

 on the steamer "Lady Havelock." Never however were these cilia seen in motion, 

 and this suspicious circumstance led to a more minute scrutiny, which revealed that 

 the appearance is due to an optical illusion. In reality the apparently ciliated layer 



M 



