xxiir STRUCTURE OF A MEDUSA 241 



outer surfaces by a thin layer of delicate cells (ect}. The 

 clapper-like organ or manubrium (Fig. 54, c and Fig. 550 

 and D', mnb) is formed of two layers of cells, precisely 

 resembling the ectoderm and endoderm of Hydra, and 

 separated by a thin mesoglcea ; it is hollow, its cavity (Fig. 

 55, D, ent. cav] opening below, i.e. at its distal or free end, 

 by a rounded aperture, the mouth (mth\ used by the medusa 

 for the ingestion of food. At its upper (attached or proxi- 

 mal) end the cavity of the manubrium is continued into four 

 narrow, radial canals (Fig. 54, c, rad. c, and Fig. 55, D and 

 D' rad} which extend through the gelatinous substance of the 

 bell 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 bell. The whole system of canals is 

 lined by a layer of cells (Fig. 55, D and D', end] continuous 

 with the inner layer or endoderm of the manubrium ; and 

 extending from one canal to another in the gelatinous sub- 

 stance of the bell, is a delicate sheet of cells, the endoderui- 

 lamella (D', end. la]. 



From the edge of the bell four pairs of tentacles (Fig. 54, 

 c and Fig. 55, D, /) are given off, one pair corresponding to 

 each radial canal, and close to the base of each tentacle is 

 a little speck of pigment (Fig. 54, oc], the ocellus or eye-spot. 

 Lastly, the margin of the bell is continued inwards into a 

 narrow circular shelf, the velum (v}. 



At first sight there appears to be very little resemblance 

 between a medusa and a hydranth, but it is really quite 

 easy to derive the one form from the other. 



Suppose a short hydranth or Hydra-like body with four 

 tentacles (Fig. 55, A, A') to have the region from which the 

 tentacles spring pulled out so as to form a hollow, trans- 

 versely extended disc (B). Next, suppose this disc to become 

 bent into the form of a cup with its concavity towards the 



R 



