274 COMPARATIVE ANATOMY 



The hydranth itself only represents the upper end of a polype 

 the rest of whose body consists of the section of the hydrocaulus 

 between it arid the hydranth next following.) When the 

 colony has attained to a certain size special reproductive 

 persons are formed as buds in the axils of the branches i.e. 

 in the angles between the hydranths and the hydrocaulus. 

 These buds give rise to tubular outgrowths called blastostyles, 

 each blastostyle being nothing more than a degenerate 

 hydriform person without mouth and tentacles. It is enclosed 

 in a special investment of the perisarc called a gonotheca ; 

 that of Obelia geniculata is shaped like a Greek urn. The 

 gonotheca and blastostyle together are sometimes called a 

 gonangium. A large number of buds is formed on the 

 sides of the blastostyle, and these, developing in a peculiar 

 manner, give rise to Medusae, which are set free and escape 

 from the terminal aperture of the gonangium into the water, 

 there to swim about as independent persons. 



The Medusa-person of Obelia is shaped like an umbrella 

 with a very short thick handle. It is quite transparent, and 

 its structure can easily be made out without dissection. The 

 upper convex surface of the umbrella is called the ex-umbrella ; 

 the lower concave surface the sub-umbrella ; the handle the 

 manubrium. The margin of the umbrella is fringed with 

 tentacles, whose number varies according to the age of the 

 medusa. At the time of liberation from sixteen to twenty four 

 are present in the medusa of O. geniculata^ and as growth 

 proceeds, the number is increased by the development of new 

 tentacles between those already existing. 



At the extremity of the manubrium is a cross-shaped opening, 

 the mouth, leading into a large gastro-vascular cavity occupying 

 the whole extent of the manubrium. This cavity is covered 

 in above by the central part of the dome of the umbrella, but 

 it gives off peripherally four radial gastro-vascular canals which 

 run at right angles to one another through the substance of 

 the umbrella and communicate with a ring canal running all 

 round its margin. The four radial canals coincide in direction 

 with the four arms of the cruciform mouth and define the 

 principal secondary or radial axes of the medusa. As is the 

 case in all the Ccelenterata, the primary or chief axis of the 

 body is defined by an imaginary line passing through the 

 mouth and the centre of the dome of the umbrella. The 



