THE MEDUSID^E. 



349 



the sight; but these shapeless objects were beautiful while 

 they moved along in their own element, and their simple 

 organisation shows no less the masterhand of the Creator 

 than the complex structure of the higher stages of animal 

 existence. With the exception of the Ctenophora, they all 

 belong to the hydrozoic class, and from the great diversity 

 of their structure have been ranged under four orders, Me- 

 dusidse, Lucernaridae, Calycophoridae, and Physophoridse. 



The Medusidse are distinguished by their globular or bell- 

 shaped disc, which by its alternate contractions and expansions 

 forces them forward through the water. By contracting the 

 whole or only part of its disc, the medusa has it in its power to 



Medusid 6een in profile, b. The same viewed from below, c. Its polypite. d. Part of its mar- 

 ginal canal, and oihpr structures in connection therewith, v. Disk or swimming organ. 

 r. Folypite. -d/. Veil. v. Tentacle. v- Radiating canal. %'• Marginal canal. 

 u. Reproductive organ, •'. Coloured spot. o". Marginal vesicle. 



direct its movements, and while thus swimming along with the 

 convex side of the disc directed forwards, and its oral lobes and 

 tentacles following behind like " streamers long and gay," it 

 may well rank among the most elegant children of the sea. 



From the roof of the disc a single polypite is suspended, 

 whose mouth, generally produced into four lobes, though in some 

 forms it is much more divided, passes into the central cavity 

 (stomach) of the swimming organ, from which canals (either 

 four in number, or multiples of four) radiate to join a circular 

 vessel surrounding the margin of the bell. A shelf-like mem- 

 brane or veil, extending around the margin, and highly contrac- 

 tile, assists locomotion by narrowing more or less the aperture 



