322 MANUAL OF NATURAL HISTORY. 



the Sea-Urchins, these likewise exhibit various forms 

 — departures from the usual stellate type. Thus we 

 have the globular Beroe rolling along the surface of 

 the sea ; the crested Physalia, with its hydrostatic 

 float ; the disk-like Guvieria ; the Rhizostoma and 

 Medusa with their umbrelliform bodies and tenta- 

 cular filaments ; the Velella with its little lateen 

 sail ; and the Cestum, like a long translucent riband* 



Next come the Rotifers, spinning through the 

 stagnant waters by means of ciliated wheel-like 

 organs, which appear to have a revolving motion, 

 — singular but minute animals, whose bodies are 

 enclosed in clear pellucid shells, and whose giz- 

 zards are armed with powerful teeth for crush- 

 ing their food, and which, moreover, possess the 

 extraordinary property of being restored to life 

 and activity after they have been completely dried 

 up and converted into dust. Belonging also to 

 that division in which the oral and anal apertures 

 are distinct, we find the phytoid Polyzoa, animals 

 so like the Polypifera as formerly to have been con- 

 founded with them. As in the true Potyps, their 

 mouth is surrounded with tentacles, but the diges- 

 tive apparatus, instead of being confined to a simple 

 stomach with an oral orifice, is produced into an ali- 

 mentary canal furnished with a distinct excretory 

 aperture ; the tentacles, moreover, are not simple as 

 in the Polypi/era, but are fringed with vibratile 

 cilia, which produce rapid and powerful currents to 

 direct the food towards the opening of the mouth. 



The class of Coelelmintha, or Cavitary-Parasites, 



