240 HYDROID POLYPES iess. 



ponding to each radial canal, and close to the base of each 

 tentacle is a little speck of pigment (Fig. 53, oc\ the ocellus 

 or eye-spot. Lastly, the margin of the umbrella is continued 

 inwards into a narrow circular shelf, the velum (?'). 



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 polype 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 

 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 of 

 four radial canals. Finally, suppose the inner and outer walls 

 of this cavity to grow towards one another and meet, thus 

 obliterating the cavity, except along four narrow radial areas 

 (d, rad) and a circular area near the edge of the umbrella 

 (d, cir. c). This would result in the substitution for the 

 continuous cavity of four radial canals opening on the one 

 hand into a circular canal and on the other into the cavity 

 of the manubrium {ent. cav), and connected with one another 

 by a membrane — the endoderm-lamella {end. la) — indi- 

 cating the former extension of the cavity. 



It follows from this that the inner and outer layers of the 

 manubrium are respectively endoderm and ectoderm : that 

 the gelatinous tissue of the umbrella is an immensely 

 thickened mesoglcea : that the layer of cells covering both 

 inner and outer surfaces of the umbrella is ectodermal : and 

 that the layer of cells lining the system of canals, together 

 with the endoderm-lamella. is endodermal. 



