TYPE CCELENTEliA. 



95 



which development ceases varies in different forms ; in Tvhu- 

 laria, for instance, the medusoid bud resembles a medusa ex- 

 cept that it lacks tentacles, sense-organs and mouth, and is 

 not free-swimming ; in Clava not only does development 

 cease at an earlier stage, but a certain amount of degeneration 



Fig. 48. — Diagrams showing the Development of a Medusa-bud. 

 A, outpushing of body-wall of polyp ; B, thickening of ectoderm ; C, forma- 

 tion of subumbrellar cavity ; D, transverse section through 0, along the 

 line indicated by ab ; E, formation of radial canals ; F, transverse section 

 through E, along the line indicated by ab. 



cc = circular canal. rs = radial pouch. 



TO = cavity of manubrium. su — subumbrellar cavity. 



TO = radial canal. v = velum. 



occurs, the rudimentary subumbrellar cavity never commu- 

 nicating with the exterior and the radiating and circular 

 canals being entirely obliterated ; and finally in Midendrium 

 the bud never develops beyond the earliest stage in which it 

 is a simple tubular outgrowth of the body-wall. 



The Relationships of the various Orders to one Another. — Since the 

 HydrarisB show such a simple type of organization it is generally supposed 

 that they represent more or less closely a primitive ancestral form, though 

 their usual habitat in fresh water suggests the possibility of their having 

 undergone some degradation. If they do represent the primitive Hydro- 



