3i8 OBELIA CHAP. 



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 polype 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 /'.., 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 

 movement is due to the contraction of the muscle-processes 

 and muscle-fibres of the sub-umbrella and velum, some of 

 which differ from the similar structures in the polype in 

 exhibiting a delicate transverse striation. 



There is still another important matter in the structure of 

 the medusa which has not been referred to. At the junction 

 of the velum with the edge of the umbrella there lies, imme- 

 diately beneath the ectoderm, a layer of peculiar, branched 

 cells, containing large nuclei and produced into long fibre- 

 like processes. These nerve-cells (pp. 167 and 303) are so 

 disposed as to form a double ring round the margin of the 

 bell, one ring (Fig. 80, D, nv) being immediately above, the 



