I9I5-] 



Fauna of the Chiïka Lake : Coelenterates. 



71 



is thick and muscular. It is capable to some extent of retraction, the margin of the 

 retracted portion being angular in vertical section. There is a minute central aboral 

 pore. 



There are twelve complete and thirty-six incomplete mesenteries, the latter 

 being situated in the intermesenterial spaces only, six in each. The two central 

 incomplete mesenteries in each space are fertile. The longitudinal muscles are very 

 feebly developed and in strictly horizontal sections the width of the mesenteries is 

 almost uniform throughout, the folds at the base on both sides in particular being 

 'diffuse and poorly developed. The two pairs of directive mesenteries are very short. 



The stomodaeum is ample and extends much more than half way down the 

 column in a state of expansion. 



_ &?v. 



?7Z-. 



FrG. 1. — Gyrostoma glaucum, sp. nov. 



A. Transverse section through the column in the lower part of the stomodaeum. 



B. Vertical section of the lower part of the sphincter. 



cm. = circular muscle: ec. = ectoderm: en. = endoderm : g. — gonad: m. = mesogloea : t.= tentacles 

 thrust into mouth. 



The column- wall is thin, the mesogloea in particular being scanty. The circular 

 muscles lie at the base of the endoderm and are not very highly developed. The 

 sphincter is little if at all differentiated from any other part of the sheath in a state 

 of contraction. When the tentacles are fully everted there is, indeed, a region just 

 below the disk in which its folds are a little stronger than in the region immediately 

 succeeding it; but similar folds may also be observed in the aboral half of the 

 column. These features are indicated by increased opacity and stronger annulation 

 of the body-wall (pi. viia, fig. 1). 



The length of the column of our largest specimen was, fully expanded, 10 mm.; 

 that of the other adults about half as much. 



Type. No. Z.E.V. 6825/7, Ind. Mus. 



