345 



CHAP. XVII. 



CCELENTEBATA. 

 POLYPS AND JELLY-FISHES. 



Thread-cells or TJrtieating Organs. — Sertulariae. — Campanulariadae. — Hydrozoio 



Acalephae Medusidse. — Lucernariadae. — • Calycophoridse. — The Velella. — The 



Portuguese Man-of-war. — Anecdote of a Prussian Sailor. — Alternating Fixed 

 and Free-swimming Generations of Hydrozoa. — Actinozoa. — Ctenophora — Their 

 Beautiful Construction. — Sea-anemones. — Dead Man's Toes. — Sea-pens. — Sea- 

 rods. — Bed Coral. — Coral Fishery. — Isis hippuris. — Tropical Lithophytes. — 

 History of the Coral Islands — Darwin's Theory of their Formation — The 

 progress of their Growth above the level of the Sea. 



Despite the low rank they occupy in the hierarchy of animal 

 life, the Ccelenterata, comprising the numerous families of the 

 Jelly-fishes and Polyps, play a most important part in the house- 

 hold of the ocean, for the sea is frequently covered for miles 

 and miles with their incalculable hosts, and whole archipelagos 

 and continents are fringed with the calcareous structures they 

 raise from the bottom of the deep. 



Their organisation is more simple than that of the preceding 

 classes, for they have neither the complex intestinal tube of the 

 polyzoa or the sea-urchins nor the jointed rays or arms of the 

 star-fishes ; their whole digestive apparatus is but a simple sac, 

 and their instincts are reduced to the mere prehension of the 

 food that the currents bring within reach of their tentacles, or 

 to the retraction of these organs when exposed to a hostile 



SLLiiBiCK.* 



But, simple as they are, they have been provided by Nature 

 with a comparatively formidable weapon in those remarkable 

 " thread-cells," or urticating organs, which are so constantly met 

 with in their integuments, and chiefly in their tentacles. 



The thread-cells are composed of a double-walled sac having 

 its open extremity produced into a short sheath terminating in 



