MEDUSA CTENOPIIOU. 



211 



FIG. 153. SHIIJU rtlu n'n\a (after Gosse). 



free pole pierced by an oral opening situated on a flat or conical 

 prominence, the oral cone. The mouth is surrounded by one or 

 more circles of tentacles, and leads into a simple cylindrical body 

 cavity (Hydroidjiolyps), or through an ossophageal tube into a compli- 

 cated gastro vascular cavity (Antliozoa, fig. 153). The disappearance 

 of the tentacles gives rise to the so-called polypoid form, which 

 consists of a simple hollow tube fur- 

 nished with a mouth. 



The Medusa type. The free-swim- 

 ming Medusa consists of a flattened 

 disc or arched bell of gelatinous or 

 cartilaginous consistence, from the under 

 surface of which hangs a central stalk, 

 the manubrium, bearing at its free 

 end the mouth. This manubrium is 

 frequently prolonged in the neighbourhood of the mouth into 

 numerous lobes and tentacles, while from the edge of the disc arise a 

 varying number of thread-like tentacles. The central cavity of the 

 body, into which the hollow manubrium leads, is called the gastric 

 cavity, and from it peripheral pouches or radial canals, the so-called 

 vessels, run to the edge of the disc, where, as a rule, they are con- 

 nected by a circular vessel. 



The movements of the Me- 

 dusa are effected by the alter- 

 nate contraction and dilatation 

 of the muscular under surface 

 of the bell, i.e. of the subuni- 

 brella. 



Rudimentary Medusae, in 

 which the manubrium and 

 marginal tentacles are absent, 

 are found. They are called 

 Medusoids, and do not acquire 

 individual independence, but 

 remain attached to the body 

 of the Medusa or Polyp from which they are budded. 



The Medusie and Polyps, in spite of the important differences 

 between them, are but modifications of the same plan of structure. 

 A Medusa may be compared to a free, flattened Polyp, possessing a 

 gastric cavity and a muscular and enlarged oral disc. 



The Ctenophor type has fundamentally the form of a sphere, 



FIG. 151. Medusa of the Podocoryne cornea with 

 four tentacles at the edge of the disc, ovaries 

 ;ind manubrium, immediately after separa- 

 tion from the stock. 



