82 CEYLON PEARL OYSTER REPORT. 



is an extremely delicate semi-mucilaginous external cuticular layer the epicuticle as 

 we may perhaps term it deriving its deceptive appearance ofciliation from numerous 

 closely-set vertical tulmli (Plate I., fig. 15). Underlying this is the thin true 

 cuticle, perforated with pores corresponding in position and continuous with the 

 tubules of the epicuticle. In consistence it is elastic, firm, and strong. 



When one of these larvae is being examined alive under pressure, after some time, 

 as activity and vitality decrease, the epicuticle begins to disintegrate, delicate 

 bladder-like cells filled with clear, colourless, oil-like globules ooze through the tubules 

 from the sub-cuticular tissues, the mucilaginous epicuticle disappears concurrently, 

 apparently dissolving, so that after the lapse of a few minutes the larva shows but 

 the merest traces of this investment. The body appears then to be bounded 

 externally by the firm and very thin true cuticle, a multitude of free thin-walled sacs 

 full of oil-globules clustering cloud-like around the larva (Plate I., fig. 15). 



In the elongated Balanoglossus-like older individuals belonging to this first larval 

 stage, the head rudiment becomes longer and more proboscis-like ; its Imlk increases 

 more rapidly than its sheath, which tends to assume a collar-like form (Plate I., 

 figs. 12 and 13). At this stage, movements become more active, and the larva, 

 facilitated by its more vermean form, crawls restlessly about when freed from its 

 capsule. Histologically, the tissues show no further differentiation. 



The larvae we have described correspond so closely with the figures of those found by 

 Monsieur Seurat given in Professor Giard's article mentioned above, that we think 

 there is little doubt that they are at least generically the same. Professor Giard 

 identifies these as belonging to the family Monobothria of van Beneden's Order 

 Pseudophvllidea. If Professor Giard's identification be correct, then our larvae also 

 belong to this family. We have, however, found a larval stage of an undoubted 

 Tetrarhynchus living in the pearl oyster, and it is not impossible, though it seems 

 improbable, that this stage is but a later one of such of the larvae described above as 

 escape entombment in a pearl. 



This second stage, or larval Tetrarhynchus, shows a great advance on the larva 

 described above. In length it measures from 4"5 millims. to 5"5 millims., and has a 

 sagittate outline. The body, covered with a sculptured cuticle, is sub-cylindrical, 

 decreasing slightly in diameter at the extreme posterior extremity (Plate II., fig. 19). 



The anterior end is sub-conical, furnished laterally with two hood-like lappets, 

 which are laid back ear-like, one on either side, when the creature is quiescent. 

 They function as organs both of progression and of adhesion. Two muscular cup- 

 shaped depressions occur in each, constituting two pairs of simple suckers (Plate II., 

 fig. 22). When at rest, these suckers are not apparent ; but as the lappets sweep 

 forwards, either as swimming or locomotor organs, or as organs of prehension and 

 attachment, the pits or suckers become conspicuous and comparatively deep. The 

 lappets are extremely mobile, changing form continuously. 



The short caudal region, slightly less in diameter than the rest of the body, 



