INVERTEBRATA. 



391 



medusa is best explained when the structure of the medusa 

 has been described. 



5. The Medusa. The medusa is about J in. in diameter 

 when full-grown. It consists (Fig. 199) of a circular disc 

 slightly campanulate or bell-like in shape : this is called 

 the umbrella. From the middle of the concave surface of 

 the umbrella hangs a short hollow process, the manubrium, 

 at the end of which is the mouth. The cavity of the 



PERRADIAL 



-ADRADIAL 



<)> INTERRADIAL 



Fig. 199. MEDUSA OF OBKLIA SEEN FROM THK VKNTHAT. on ORAL SIDK. 

 The figures 1 to 8 are placed near the bases of the adradial tentacles containing the 

 statocysts. The gonads are seen on the radial canals and the manubrium in the 

 centre. 



manubrium leads into a central cavity in the umbrella, 

 and from this radiate four radial canals which open into a 

 circular canal running near the margin of the umbrella. 

 The cavities and canals are, of course, lined with endoderm, 

 but the endoderm is not confined to them. In reality the 

 umbrella is divided into two layers, a dorsal and ventral, 

 or upper and lower, by a continuous double layer of en- 

 doderm (Fig. 200). The upper layer is called the exuin- 

 brella, the lower the subumbrella. The two layers of 



