THE CEYLON PEARL-OYSTER. 291 



MyzorhyncJms uniformly muscular, without obvious division 

 into muscular tracts; retractile within an annular collar; in 

 section it may appear either conical, lenticular, or flattened, 

 concave and sucker-like ; protrudes as a conical papilla when in 

 locomotion. This anterior muscular region, including the collar, 

 is about one-third of the total length of the larva when extended. 

 The whole myzorhynchus can be protruded, the collar then 

 forming an annulus around it. 



Collar or cephalic sheath muscular with denticulated cuticle, 

 the denticles tricuspid. (Herdman's Report, Part II. (Shipley & 

 Hornell, Parasites of the Pearl-Oyster) pi. i. figs. 10, 11, <k 14, 

 pi. ii. fig. 18 ; also Pt. V. (Pearl -Production) pi. iii. fig. 9.) 

 The denticles measure fi'om 3^ to 5/^^ in diameter. 

 Hinder part of the larva centrally parenchymatous, the 

 parenchyma containing the calcareous corpuscles characteristic 

 of Cestode larvje, peripherally more muscular. 



The hinder part of the "body is covered by a thick, radially 

 marked epicuticle, permeated by numerous closely-set tubuli, 

 and suggesting on superficial examination a coat of cilia. This' 

 epicuticle varies in thickness but is generally about O'OS mm. 

 thick, and the true cuticle lies under it. 



This form is distinguished from the next described worm by its 

 larger size (Herdman gives the size as about six times that of the 

 smaller form), the undivided musculature of the myzorhynchus, 

 and the wider and more open character of the collar-sheath of the 

 myzorhynchus in the resting-stage. 



Habit. Resting in spherical fibrous cysts, derived from the 

 connective tissue of the host, in the Ceylon Pearl-Oyster, 

 Margaritifera vulgaris. Most frequent in the visceral mass, 

 notably the liver. 



Habitat. Gulf of Manaar (Herdman c6 Hornell). Trincomalee 

 ( Willey). 



The following is a description of a worm which I ref^ard as 

 in all probability the adult of this larva. The single specimen 

 was obtained from the spiral intestine of Aetobatis narinari, by 

 Mr. Hornell, on 4th January 1905, and had apparently been 

 overlooked by Mr. Shipley among some duplicate specimens of 

 Kystocephalus translucens, along with which I found it when 

 examining Dr. Shipley's material. After it had been cleared and 

 examined as a transparent object, Dr. Shipley very kindly allowed 

 me to have sections cut from it to compare with those of the 

 larva in the pearl-oyster. 



(?) Adult of Tylocejjhalum ludijicans, sp. n. (Pis. XLVI. <jk 

 XLVII. figs. 60-64.) Length 12 mm. Head 0*6 mm. long by 

 0-5 mm. broad ; pyriform, slightly broader in front than behind ; 

 transition from head to neck not very sharply defined. The 

 myzorhynchus in this sjjecimen is retracted within its sheath, as 

 is usually the case with the larva in the pearl-oyster ; it is about 

 •3 mm. in diameter. Around the head are four marginal suckers 

 about -125 mm. in diameter. Proglottides about 140 in number, 

 increasing but little in breadth from before backwards; they 



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