TRACHYMEDUS^ — OLINDIOIDES. 



359 



are constantly contracting into helical coils and then expanding, and it is apparently their 

 office to capture prey. These contractile tentacles grow out from the marginal bulbs; and 

 indeed, the marginal bulbs are only embryonic or rudimentary velar tentacles. 



There are 2 lithocysts at the base of the entodermal core of each exumbrella tentacle. 

 Each lithocyst is a closed vesicle, buried in the gelatinous substance on the outer side of the 

 ring-canal at a short distance above the bell-margin. Each lithocyst contains a single, refrac- 

 tive, spherical concretion, which is suspended in the cavity of the vesicle and is supported 

 by and contained within a club-like mass of cells. The lithocyst and concretion are accord- 

 ing to Goto of ectodermal origin, but this may seem improbable in view of their known ento- 

 dermal origin in the closely allied genus Gomonemus. 



Fig. iOT,.—Olindhides formosa, after Goto, in Mark Anniversary Volume. Showing sector of bell, part of bell-margin, 

 and two views showing mode of origin of 6 radial-canals from manubrium. 



The manubrium is 4-sided, prismatic, with 4 simple, somewhat flaring lips. 4 canals 

 arise from the 4 corners of the stomach, but 2 of these primary canals bifurcate immediately 

 after leaving the stomach so that there are 6 radial-canals which reach the circular vessel 

 6o° apart. The ring-canal is broad and gives rise to numerous blindly ending, centripetal 



