1915.] Fauna of the Chilka Lake : Coelenterates. 77 



but that the condition of these folds varies considerably in different stages of expan- 

 sion and contraction of the column. There are as a rule no independent muscle- 

 fibres in this region, but in one specimen (fig. 2) I have found a few widely separated 

 in the mesogloea and without any definite arrangement. When the disk is retracted 

 but the column expanded horizontally, the differentiated region of the muscle-sheath 

 extends over a considerable area ; part of it is introverted, while part still remains 

 external. 



The facts relating to expansion and contraction of the column and to retraction 

 of the tentacles were not fully understood by me in 1907. When the animal is left 

 dry by the retreat of the tide its tentacles are always 

 retracted, but its column fully expanded. The oral disk /"' /f'Mf 



is withdrawn for some distance into the column and the 

 walls of the latter are partially closed by a constric- 1 ^jg 



tion above the tips of the tentacles, but as a rule a /^ / ilk ,e?K 



small opening remains patent. In this condition the pffg I Jm 



actual body- wall is much swollen and remarkably trans- f ^jlff 



lucent. If the animal is touched, water is squirted \ \\^S| 



violently from the orifice above the tentacles. I was _ .-■■■-" "X X\?MkxA 

 wrong, however, in thinking (1907, p. 64) that any 

 part of this water was contained between the layers 

 of the wall or in its mesogloea. In specimens killed *« -\ ' 



with the body fully expanded — this is easily accom- 

 plished by pouring boiling formalin upon them in FlG - ^.—Metndium schtllenanum 

 a small dish — the column wall will be found to be 



very thin and to be expanded by liquid within the , er lca sec lon roug 



J ... sphincter. 



mesenterial chambers. The mesenterial filaments lie 



ec. = ectoderm : e». = endoderm. 



bathed in this water in the middle of the body-cavity. 



A sudden contraction of the circular muscles of the column causes part of the water 

 to be shot violently out of the mouth and consequently out of the orifice lying 

 immediately above it. 



When the tentacles are fully retracted the whole of the visible part of the 

 column is smoothly rounded, but as they are extruded a distinct convex ring' makes 

 its appearance round the upper extremity. When the oral disk has been completely 

 extruded the column itself contracts strongly both in a transverse and a vertical 

 direction, becoming relatively short and slender. This is due partly to muscular action 

 and partly to the fact that water is expelled from the body-cavity. In the living 

 animal the column in this condition is more or less completely hidden by the 

 tentacles, but if a specimen is bisected vertically a very distinct fold of the 

 body- wall can be seen (pi. viia, fig. 2) some little distance below the base of the 

 tentacles. It is in this fold that the circular muscle of the column is most distinctly 

 strengthened and differentiated. 



See Stoliczka, 1869, pi. x, fig. 6. 



