80 CEYLON PEARL OYSTER REPORT. 



In the liver and in the base of the gills they usually reach the greatest size ; 

 in the gill-filaments, where they occasionally abound, they seldom attain other than 

 minute dimensions. Each is enveloped in a tough, elastic, and fibrous capsule of 

 spherical form, derived from the adjacent connective-tissue cells (Plate L, figs. 5, 6, 

 and 8). The capsules of those found in the liver and gill-bases are specially thick 

 and strong, and slightly opalescent. In extreme cases the fibrous capsule has a 

 thickness equal to fully half the diameter of the enclosed parasite (Plate I., fig. 5 a, 

 and fig. 8 a and b). 



The minute larvae encapsuled in the gill-filaments and these are easy of observa- 

 tion because of the transparency of the capsule membrane (Plate I., figs. 5 and 8) 

 may be seen occasionally to rotate slowly within their prison. The body is sub- 

 globular and remains so, even after considerable increase in size. The oldest larvae 

 however tend gradually to assume an elongated cylindrical form, with a pointed 

 protrusible head, a denticulated collar, and a long oval body, foreshadowing the 

 change to the rarer second or hooded larval phase (Plate I., figs. 12 and 13). 



The larva in this stage closely resembles one of those figured in a paper by 

 Professor Giard on " L'Origine Parasitaire des Perles."* It was observed by 

 Monsieur Setjrat, who not unnaturally mistook it for Amphistoma. 



In the globular stage, whether minute or comparatively large, the structure differs 

 but in details. Thus the body consists of two regions, an anterior, the smaller, and a 

 large bladder-like posterior division. The former measures approximately but one- 

 third of the total diameter. Viewed en face, this region has the appearance of a 

 broad convexly-annular sucker, with a wide central orifice, wherefrom protrudes 

 slightly the rounded summit of a low eminence (Plate I., fig. 9). 



The saccate hinder portion of the body is thin-walled and filled with granular 

 contents wherein rounded refractile corpuscles lie scattered abundantly, especially in 

 the peripheral layer (Plate I., tigs. 12 and 13). Under slight pressure, as first seen, 

 it exhibited a striking resemblance to a tiny Trematode, or it might be mistaken for 

 a large Gregarine. 



When freed from its investing membranes, the larva progresses slowly by means of 

 the alternate elongation and contraction of the body, the low central anterior 

 eminence assisting by protrusion and retraction. The anterior region is clear and 

 slightly tinged with yellow ; the hinder is colourless and granular. 



The cephalic " sucker " is in reality a head-sheath, the low median eminence 

 within being a proboscis-like head rudiment, the scolex-rudiment (Plate I., fig. 6). 

 When the latter is retracted the normal condition when at rest within the cyst 

 capsule it lies sunk within the sheath, separated from it by a deep and narrow 

 encircling groove. Under the influence of slight pressure, the head is seen (Plate I., 

 fig. 11) to be shot out in the form of a blunt cone, the head groove being temporarily 

 obliterated. The muscular walls of this cephalic apparatus, sheath as well as head, 



* 'C. R. Soc. Biol.,' LV., 1903, p. 1222. 



