310 BOUGAINVILLEA chap. 



of four radial canals. Finally, suppo.se 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 {dr. 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 (d' end. 

 lam) — indicating 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 mesogloea : 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. 



Thus the medusa and the hydranth are similarly con- 

 structed or homologous structures (p. 217), and the hydroid 

 colony is dimorphic (p. 250), bearing zooids of two kinds. 

 In some allied forms, this individuation may go still further, 

 the zooids being of very various forms and performing 

 diverse functions : such a colony is said to be polymorphic. 



Sooner or later the medusae separate from the hydroid 

 colony and begin a free existence. Under these circum- 

 stances the rhythmical contraction — i.e. contraction taking 

 place at regular intervals — of the muscles of the umbrella 

 causes an alternate contraction and expansion of the whole 

 organ, so that water is alternately pumped out of and drawn 

 into it. The obvious result of this is that the medusa is 

 propelled through the water by a series of jerks. The 

 rrjovement is performed by means of the muscle-processes 



