CHAP. IV MEDUSA 309 



the enteric cavity being continued into the unabrella (**«/. caif) ; the hypostome 

 now forms a manubrium (^mni), 



C, transverse section of the same through the plane a b, showing the continuous 

 cavity {ettt. cav') in the umbrella. 



D, fully formed medusa ; the cavity in the umbrella is reduced to the radial 

 {rod) and circular {cir. c) canals, the velum {v) is formed, and a double nerve- 

 ring {nv, nv') is produced from the ectoderm. 



D', transverse section of the same through the plane a i, showing the four radial 

 canals (rati) united by the endoderm-lamella (etui, lam), produced by partial 

 obliteration of the continuous cavity (enf. cav') in C. (From Parker's Biology.) 



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. The whole system of canals is lined by a 

 layer of cells (Fig. 77, 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 endoderm-lamella 

 (d', end. lam). 



From the edge of the umbrella four pairs of tentacles 

 (Figs. 76, c and 77, /) 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. 76, oc), the ocellus or eye- 

 spot. Lastly, the margin of the umbrella iscontinued 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 simple polyp or Hydra-like body with four 

 tentacles (Fig. 77, 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 

 hypostome, and to undergo a great thickening of its meso- 

 gloea. A form would be produced like c, i.e. a medusa-like 

 body with umbrella and manubrium, but with a continuous 

 cavity (c', ent. cav') in the thickness of the umbrella instead 



