PARTIAL TRIPLOBLASTIC CONDITION 241 



like Zoothamnium (p. 134), is dimorphic, bearing zooids of 

 two kinds. 



The ectoderm cells of the hydranth bear muscle-processes 

 like those of Hydra (p. 225, Fig. 52, c) : in the medusae 

 similar processes are found on the inner concave side of the 

 bell and in the velum. Sometimes, however, the place of 

 these processes is taken by a layer of spindle-shaped fibres 

 (Fig. 56, A), many times longer than broad, and provided 

 each with a nucleus. Such muscle-fibres are obviously cells 

 greatly extended in length, so that the ectoderm cell of 

 Hydra with its continuous muscle-firvtess is here represented 

 by an ectoderm cell with an adjacent muscle-^//. We 

 thus get a partial intermediate layer of cells between 

 the ectoderm and endoderm, in addition to the gelatinous 

 mesoglcea, and so, while a hydroid polyp is, like Hydra, 

 diploblastic (p. 223), it shows a tendency towards the as- 

 sumption of a three-layered or triploblastic condition. 1 Both 

 the muscle processes and muscle-cells of the medusae differ 

 from those of the hydranths in exhibiting a delicate 

 transverse striation (Fig. 56). 



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 bell 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. 



There is still another important matter in the structure ot 

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



1 This intermediate layer is not, however, exactly comparable to the 

 mesoderm of higher animals (p. 274). 



R 



