318 OBELI A CHAP. 



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 ecto- 

 dermal : and that the layer of cells lining the system of 

 canals, together with the endoderm-lamella, is endo- 

 dermal. 



Thus the medusa, the polype, and the blastostyle are 

 similarly constructed or homologous structures (p. 217), 

 and the hydroid colony is trimorphic (compare p. 250), 

 bearing zooids of three 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 cir- 

 cumstances 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 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, immediately 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 



