210 



CEYLON PEARL OYSTER REPORT. 



Srarn 



Nasal 

 coecum 



description given by Kyle; text-fig. 1, which is part of a transverse section through 

 the head of a young specimen of C. bengalensis, shows the relations of the nasal 

 coecum to the mouth. It is a large thin-walled sac, triangular in transverse section, 



lying dorsally to the mouth and only separated from 

 the latter by the buccal epithelium and its own wall. 

 It extends backwards for a considerable distance. 

 Anteriorly it is formed, as Kyle's figures show, by 

 the union of two lateral, relatively narrow canals, 

 one proceeding from each olfactory sac. These pass 

 downwards and fuse together over the roof of the 

 mouth. 



The perforation joining the cavity of this sac with 

 that of the mouth lay apparently in Kyle's specimen 

 about the middle of the length of the coecum. 



The occurrence of this opening in only one specimen 

 of a number examined suggests that it is cpiite 

 abnormal, and most probably the result of an accident. 

 -Mouth I n oue specimen, C. brevirostris, which was cut into 

 serial sections, a Copepod parasite possessing convo- 

 luted egg sacs was found in the ccecum. Several 

 other larger specimens were then dissected, and in one 

 of these (C. brachyrhipichus) a single Copepod parasite 

 Avas found, which is described in Part I. of this Report 

 (p. 294) as Chonclracanthus cynoglottidis, Thompson 

 and Scott. The anchor hooks of this Copepod were 

 embedded in the muscles outside the dorsal wall of the 

 coecum, and its body lay on the thin ventral wall, so 

 that it was immediately apparent on opening the buccal cavity sufficiently to show 

 the roof of the mouth. 



The occurrence of this Copepod in such a situation suggests what is doubtless the 

 real -explanation of the naso-pharyngeal communication that is, that the perforation 

 was an injury due to this or a similar parasite. Either the body and egg sacs resting 

 on the thin ventral wall of the coecum had produced the rupture of the latter, or the 

 anchor hooks had themselves become embedded in this part of the wall and, as the 

 parasite increased in size, broke through it. With such an origin the perforation 

 would have no morphological significance whatever ; and the speculations of its 

 discoverer fin the origin of the internal nares of vertebrata, which are based on its 

 occurrence, are therefore without real value. 



Text-fig. 1. Transverse section 

 through head of Cynoglossus 

 bengalensis. x about 12 dia- 

 meters. 



Rhomboidichthys azureus, Alcock. 

 Two specimens answer to the description of this species, except that the number of 



