v MEDUSAE 315 



(nith), used by the medusa for the ingestion of food. 

 Very commonly as the medusa swims the umbrella 

 becomes turned inside out, the sub-umbrella then 

 forming the convex surface and the manubrium spring- 

 ing from its apex (Fig. 78, C). At its upper (attached 

 or proximal) end the cavity of the manubrium is con- 

 tinued into four narrow, radial canals (Figs. 78, B, D, 

 and 79, rad. c, and Fig. 80, D, and D', rad) which 

 extend through the gelatinous substance of the umbrella 

 at equal distances from one another, like four meridians, 

 and finally open into a circular canal (dr. c) which runs 

 round the edge of the umbrella. By means of this 

 system of canals the food, taken in at the mouth and 

 digested in the manubrium, is distributed to the entire 

 medusa. The canals are lined by a layer of cells 

 (Fig. 80, D and D', end) continuous with the inner layer or 

 tndoderm of the manubrium ; and extending from one 

 canal to another, in the gelatinous substance of the 

 umbrella, is a delicate sheet of cells, the endoderm-lamella 

 (D', end. lam). 



The edge of the umbrella is produced into a very 

 narrow fold or shelf, the velum (Fig. 79, vl, Fig. 80, v), and 

 gives off the tentacles (/), which are sixteen in number in 

 the newly-born medusa, very numerous in the adult. At 

 the bases of eight of the tentacles two in each quadrant 

 are minute globular sacs (/), each containing a cal- 

 careous particle or lithite. These are the marginal sense- 

 organs or lithocysts : they were formerly considered to be 

 organs of hearing, and are hence frequently called 



otocysts " : in all probability their function is to guide 

 the medusa by enabling it to judge of the direction in 

 which it is swimming. The marginal organs in this case 

 |!may therefore be looked upon as organs of the sense of 

 Direction or of equilibration, and may be spoken of as 

 statocysts (compare p. 189). The velum consists of a 



