v MEDUSA 315 



the mouth (mth\ 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 springing from its 

 apex (Fig. 78, c). At its upper (attached or proximal) end 

 the cavity of the manubrium is continued 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 (cir. 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 endoderm of the 

 manubrium ; and extending from one canal to another in 

 the gelatinous substance of the umbrella, is a delicate 

 sheet of cells, the endo derm-lamella (D', end. lam). 



The edge of the umbrella is produced into a very narrow 

 fold or shelf, \hevelum (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 calcareous 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 

 middle layer of mesogloea with ectoderm on either side : 



