CHAP. IV MEDUS.Ii 309 



the enteric ca\ ity being contitmed Into [lie untl'vella {ejtt. caz'') ', ihe hypnitomc 

 now forms a manubrium (mn/'). 



C, transverse section of the same through the plane eti, showing the continuous 

 cavity {ent. tav' ) in the umbrella. 



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

 {rad) and circular {cir. c) canals, the velum (r>) is formed, and a double nerve- 

 ring (wz', «?'') is produced from the ectoderm. 



D', transverse section of the same through the plane a t, showing the four radial 

 canals l^rad) united by the endoderm-lamella {end. latn), produced by partial 

 obliteration of the continuous cavity {e/it. 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-latnella 

 (d', end. lam). 



From the edge of the umbrella four pairs of tentacles 

 (Figs. 76, c and 77, t) 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 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 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 



