A HYDROID MEDUSA. 



For this purpose the form Gonionemus, common at Wood's 

 Hole, is most available; in default other bell-shaped forms 

 may be used. As there is no dissection to be done, the speci- 

 mens may be used year after year, if care be taken that they 

 be not injured. 



Notice that the body is somewhat umbrella-shaped, 

 and that you can distinguish a convex or exumbrella 

 from a concave or subumbrella surface. From the centre 

 of the subumbrella projects a proboscis or manubrium 

 comparable to the handle of the umbrella and bearing 

 the mouth at its extremity. What is the shape of the 

 mouth? 



Trace the digestive or gastral cavity from the mouth 

 through the proboscis to the centre of the umbrella and 

 thence note the four (more in some species) of radial 

 canals extending out to the margin of the disc, where they 

 empty into a ring-canal. Notice the tentacles attached to 

 the margin of the umbrella. Are they the same in num- 

 ber in each quadrant? Are any of them opposite a radial 

 canal? Find the reproductive organs on the radial canals. 

 Sketch the medusa from the exumbrellar side. 



Examine it now from the subumbrellar surface and note 

 that the opening of the umbrella is contracted by a thin 

 membrane, the velum, with an opening in the centre. 

 Make a diagrammatic section of a medusa showing manu- 

 brium, radial and ring canals, tentacles and velum. 



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