I9I5-] 



Fauna of the Chilka Lake ; Coelent er ates. 



83 



and that almost the whole of the external surface of the column is covered with 

 minute papillae. The internal structure of the two species is also very similar, 

 except that P. chilkaeus has a true mesogloeal sphincter situated a short distance 

 below the oral disk. This species is also more sensitive to drugs than its Gangetic 

 ally and therefore much more difficult to preserve in a natural condition. Conse- 

 quently, preserved specimens of the two look as a rule very different {cf. figs. 3 

 and 4, pi. viiaj. Those of P. chilkaeus are darker in colour, being of a glaucous grey 

 shade, and, owing to the strong contraction of the circular muscles, much more 

 opaque ; the column is elongated and cylindrical for the most part and the physa-like 

 appearance of its aboral extremity exaggerated, while the tentacles are reduced to 

 mere knobs, though in full expansion they are as long and slender as those of 

 P. gangeticus. The position of the sphincter is clearly indicated externally by a 

 convex annulus (pi. viia, fig. 4). 



The specific characters in which the two species differ may be tabulated thus : — 





P. gangeticus. 



P. chilkaeus. 



Tentacles 



50-65. Never contracted to 

 mere knobs. Concolorous. 



2T-24- Liable to be contracted to mere 

 knobs. Sometimes with dark angulate 

 rings and a dark tip. 



Surface of column 



Smooth 



Covered, except at the aboral extremity, 

 with minute tubercles and bearing a con- 

 vex annulus a short distance below the 

 disk. 



Sphincter 



Absent 



Well developed, with elongate muscle- 

 spaces. 



Body-wall 



Very thin 



Much thicker, at any rate in a state of con- 

 traction. 



The tentacles are normally 24 in P. chilkaeus, but one or more may be aborted 

 and I have examined a specimen in which there were only 21. Even in specimens 

 preserved in alcohol the darkening of their tips occasionally persists, though the dark 

 angulate rings disappear rapidly. The markings are due to accumulations of minute 

 algae in the cells of the endoderm. In preserved specimens there appear to be two 

 concentric circles of tentacles arranged alternately, but in the living animal they 

 are distinctly grouped in threes with a single tentacle between each triad. The 

 extreme contraction to be noted in most of our examples took place before death and 

 was apparently due to the fact that unsuccessful attempts were made to paralyse the 

 animals with drugs. 



The minute papillae on the surface are produced by swellings of the mesogloea 



