THE MODE OF GROWTH OF THE RADIATES. 



HI. THE ACTINOZOA. 



The sea anemones and coral polypes are more highly developed 

 than the Hydroids, since the mouth opens into a double digestive 

 cavity, which is supported for its whole length by the six primary 

 curtains or septa. The second and lower half of the cavity en- 

 larges greatly, and communicates with the general cavity of the 

 body, the upper portion being entire, tubular, and forming a sort 

 of throat opening into the proper digestive cavity. In the Hy- 

 droids, the digestive cavity, it may be remembered, is simply hol- 

 lowed out of the body cavity and is a more primitive affair than 

 that of the Actiniae. 



While in the Hydroids also the ovaries hang outside the body 

 cavity, in the true polypes they are attached to the septa or walls 

 of the radiating chambers, so that the eggs, when ripe, drop down 

 into the body cavity, whence they pass out through the mouth, or, 

 as observed by Lacaze-Duthiers, in the coral polypes through the 

 tentacles. The chambers between the septa correspond to the 

 water canals, or chymiferous tubes of the Hydroids. 



In the coral polypes the coral is secreted in the chambe 

 there are soft partitions alternating with the limestone ones. The 

 tentacles which surround the mouth vary greatly in number. They 

 are hollow, each communicating with a chamber. 



The polypes are divided into (1), the Actinoids (Zoantharia) 

 which either secrete no limestone, as in the sea anemones, or form 

 a coral stock, as in the coral polypes, and have an indefinite num- 

 ber of tentacles, and (2), the Halcyonoids, in which the tentacles 

 are eight in number. Such are the sea fans (Gorgonia) and Hal- 

 cyonium, which does not secrete a coral stock. 



Development. The life history of a polype is soon told. Natu- 

 ralists are indebted to the magnificent memoirs of Lacaze-Duthiers 

 for a full biography of not only several genera of sea anemones 

 {Actinia mes< ' ntfrr^ndli^muhi, I'.unndes mid Sa^ariia) but also of 

 the Gorgonia, Halcyonium, red coral, and the Astraeoides, a Medi- 

 terranean form allied to Astraea. 

 (218) 



