84 CEYLON PEARL OYSTER REPORT. 



Macerated specimens and sections show that the teeth of these two Tetrarlnjnelu 

 are very different in shape, size and number. In T. halistidis the teeth are few in 

 number, slightly curved or hooked, with a well pronounced broad base (Plate II., 

 figs. 33 and 35). Seen in optical section, there are but two teeth visible in a 

 transverse row, and probably each transverse ring of teeth consists of but four to 

 six of these structures (Plate II., fig. 34). The teeth of T. pinna' are, on the other 

 hand, exceptionally numerous. They arise in two hemicircles, and the centre 

 of each hemicircle lies posterior to the free ends. The right and left hemicircles 

 alternate, as is seen in Plate II., fig. 37. The teeth are so numerous that the 

 oblique N^ shape in which they are arranged is not evident when the teeth are 

 themselves examined. Then only a forest of fine blades is seen like the clashing 

 swords of an army acclaiming an Emperor, but if the lens be focussed on to the 

 bases from which the teeth originate the orderly arrangement, as of soldiers drawn 

 up in rank, becomes apparent. The teeth of this form differ from the stout teeth 

 of T. halistidis not only in their number, but in their size and shape. They are 

 smaller and far more delicate in outline, and are shaped like a Malay kriss (Plate II., 

 fig. 36). At the proximal end is a small haft or handle, which probably represents 

 the portion embedded in the flesh. 



T. halistidis occurs encysted in the liver, and beneath the peritoneum of the 

 spotted file or trigger fishes (Balistes stelhdiis and B. mitis). Its length is about 

 12 millims. to 13 millims., and its breadth 1*5 millims. to 2 millims. It consists of a 

 rounded triangular head and an elongated, melon-seed-shaped body. The head is 

 enveloped in a protective sheath like an amnion but remaining open at a median pore 

 (Plate II., fig. 24). The hooked introverts and their muscular sheathes are all 

 confined to the head, and are enveloped in the sheath. Traces of the bothria or 

 suckers are visible. The body is quite free from the sheath and in no part 

 surrounded by it. Both body and head are crowded with calcareous corpuscles. 



What we take to be a later stage of T. halistidis also occurs beneath the peritoneum 

 of Balistes mitis and B. stellatus. This stage is represented by the squarish head 

 of a Tetrarhynchus with the introverts protruding and the bothria well marked 

 (Plate II., fig. 25). This form has thrown off its body aud its amnion, in fact the 

 whole vesicle has disappeared. It shows distinct signs of 4 bothria, such as exist in 

 Tetrarhynchus bisulcatus, Linton. The introverts and their muscular sacs lie one 

 on either side of the level of the bothria. The small papilla or protuberance at 

 the posterior end indicates the beginning of the body or string of proglottides ot 

 the adult. 



We have thus in Balistes mitis and B. stellatus in all probability two stages ot 

 the species T. halistidis. 



T. pinna? lies in more definite cysts formed by the pathological growth of the 

 tissues of its host, and from which they can be easily shelled. The cysts are 

 elongated oval or sausage-shaped, whitish or brownish-yellow in colour, and some 



