242 HYDROID POLYPES LESS. 



A', transverse section of the same through the plane a b. 



B, the tentacular region is extended into a hollow disc. 



C, the tentacular region has been further extended and bent into a 

 bell-like form, the enteric cavity being continued into the umbrella 

 (ent. cav') : the hypostome now forms a manubrium (mtib). 



c', transverse section of the same through the plane a b, showing the 

 continuous cavity (ent. cav') in the umbrella. 



D, fully formed medusa : the cavity in the umbrella is reduced to the 

 radiating (rad) and circular (cir. c) canals, the velum (v) is formed, and 

 a double nerve-ring (nv, nv) is produced from the ectoderm. 



D', transverse section of the same through the plane a &, showing the 

 four radiating canals (rad) united by the endoderm-lamella (end. /a), 

 produced by partial obliteration of the continuous cavity ent. cav' in C' 



Thus the medusa and the hydranth are similarly con- 

 structed or homologous structures, and the hydroid colony, 

 like Zoothamnium (p. 136), is dimorphic, bearing zooids of 

 two kinds. 



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 pro- 

 pelled through the water by a series of jerks. The movement 

 is performed by means of the muscle-processes and muscle- 

 fibres of the sub-umbrella and velum, both of which differ 

 from the similar structures in the hydranth in exhibiting a 

 delicate transverse striation (Fig. 57). 



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 bell there lies, imme- 

 diately beneath the ectoderm, a layer of peculiar branched 

 cells (Fig. 56, B, n. c), containing large nuclei and produced 

 into long fibre-like processes. These nerve-cells (see p. 227) 



