THE MEDUSID. 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 Ctenephora, they all 
belong to the hydrozoic class, and from the great diversity 
of their structure have been ranged under four orders, Me- 
duside, Lucernarid#, Calycophoride, and Physophoride. 
The Medusidee are distinguished by their globular or bell- 
shaped dise, 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 seen in profile. 5. Thesameviewed from below. c. Its polypite. d. Part of its mar- 
ginal canal, and other structures in connection therewith. y. Disk or swimming organ. 
zx. Polypite. J. Veil. z. Tentacle. y. Radiating canal. y’. Marginal canal. 
w. Reproductive organ. 0’. Coloured spot. 0’. 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 dise 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 contrae- 
tile, assists locomotion by narrowing more or less the aperture 
