HYDROZOA 



213 



snaky fashion, whence the name medusa. Circular striped muscles 

 run on the subumbrellar side of bell and velum, causing the characteristic 

 motion. By their contraction the bell becomes more arched and narrowed, 

 while the velum (which hangs down when at rest fig. 175, A) contracts 

 like a diaphragm across the mouth of the bell (B). Since water is thus 

 forced out through the opening the medusa is forced forward by the reac- 

 tion. The circular muscles of the umbrella and velum are separated 



FIG. 176. Tiara pilcata (after Haeckel, from Hatschek). 



by two nerve rings, one subumbrclhv, the other cxumbrellar in position 

 (fig. 177, n 1 , ir), the first supplying the muscle plexus, the other the 

 sensory organs eyes of the simplest type, red pigment spots with or with- 

 out a lens; and open or closed statocysts ('ears'). Tactile hairs are 

 abundant on the tentacles. 



The statocysts are of two types, both beginning as open organs and reaching 

 their highest development as closed vesicles. One type, the tentacular organs, 

 occurs in the Trachymedusae (fig. 177, 1-4) the other, or velar organ, in the 

 Leptomedusaj (5-6).' The tentacular organs are modified tentacles, the ento- 



