252 COELENTERATA HYDROZOA chap. 



called oral, and the opposite surface is called aboral. The cavity 

 partly enclosed by the oral aspect of the body when it is cup- 

 or bell-shaped is called the " sub-iimbrellar cavity." 



In the medusome of nearly all Hydrozoa tliere is a narrow 

 shelf projecting inwards from the margin of the disc and guard- 

 ing the opening of the sub-umbrellar cavity, called the "velum." 

 The mouth leads through the manubrium into a flattened 

 part of the coeleuteric cavity, which is usually called the gastric 

 cavity, and from this a number of 

 canals pass radially tlirough the meso- 

 gloea to join a circular canal or ring- 

 canal at the margin of tlie umbrella. 



A special and important feature of 

 the medusome is the presence of sense- 

 organs called the " ocelli " and " stato- 

 FiG. 126.-Diagram of a vertical cysts," situated at the margin of the 

 section through a medusome. umbrella or at the base of the 



cod, Coeleuteron ; M, mouth ; , , ■■ 



Man, manubrium ; R, radial tentacles. 



canal; r, ring or circular canal; The OCcUi may USUally be reCOg- 



T, tentacle ; v, velum. . ^ '^ ^ , / ° 



nised as opaque red or blue spots on 

 the bases of the tentacles, in marked contrast to their trans- 

 parent surroundings. The ocellus may consist simply of a 

 cluster of pigmented cells, or may be further differentiated as a 

 cup of pigmented cells filled with a spherical tliickening of tlie 

 cuticle to form a lens. The exact function of tlie ocelli may not 

 be fully understood, but there can be little doubt that they are 

 light -perceiving organs. 



The function of the sense-organs known as statocysts, how- 

 ever, has not yet been so satisfactorily determined. They were 

 formerly thouglit to be auditory organs, and were called 

 " otocysts," but it appears now tliat it is impossible on physical 

 grounds for these organs to be used for the perception of 

 the waves of sound in water. It is more probable that they 

 are organs of the static function, tliat is, the function of the per- 

 ception of the position of the body in space, and tliey are 

 consequently called statocysts. In the Leptomedusae each 

 statocyst consists of a small vesicle in the mesogloea at tlie 

 margin of tlie umbrella, containing a hard, stony body called 

 the " statolith." In Geryonia and some other Trachomedusae the 

 statolith is carried by a short tentacular process, the " statorhab," 



