116 



LOWER INVERTEBRATES. 



the " sea-fans," " searpens," and their allies in the second. Anatomically, the two 

 groups are distinguished, in part, as follows : In the Actinoid corals we find a large 

 number of internal radial septa and numerous external tentacula about the mouth. 

 When the number of these organs is small they are generally in multiples of six, and 

 in most instances there are no lateral appendages to the tentacles. When hard matter 

 is secreted in the tissues it is commonly in the form of carbonate of lime. The second 

 great group of Actinozoa, called the Halcyonoida, differs from the former in the 

 possession of eight, or a multiple of eight, tentacles and body septa, while the former 

 almost universally bear side branches and appendages. In those genera where a 

 skeleton exists it is tough and elastic, oftentimes of very great hardness, as in the 

 genus Isis and the well-known ornamental coral of commerce. 



Oedek I. — ZOANTHARIA. 



The so-called Actinaria, which are referred to the Actinoid corals, include a large 

 number of interesting genera. As a genei-al thing, these genera are solitary in their 

 mode of life, and often reach a great size. One of the best known genera of the 

 Actinaria is the genus Metridium, of which we have already spoken. 



Many of the Actinaria are either 

 free-swimming in their adult form, are 

 parasitic, or live with their bodies par- 

 tially hidden in the sand ; still others 

 are attached to the ground. They do 

 not, as a rule, secrete a calcareous or 

 horny skeleton, and their bodies are 

 usually very soft, without even the 

 needle-like spicules which occur in 

 the'soft forms of the Halcyonoida. 



One of the most interesting of the 

 Actinaria is the genus Edwardsia, 

 which is not attached to the ground, 

 but lives in the sand in its adult form, 

 while in younger conditions it is free- 

 swimming, even after the tentacles 

 have reached a considerable size. The 

 young Edwardsia was at first mis- 

 taken for the adult condition of an 

 Actinoid coral, and was described 

 under the name of Arachnactis. 

 Later, however, it was shown to be 

 simply the free-moving young of the 

 genus JSdwardsia. Peachia, another 

 genus of Actinaria, is parasitic in the 

 mouth-folds of the discophorous jelly- 

 fish, Cyanea. 



The Zoantharia, which are reef- 

 building corals, are closely related to the Actinaria, • and although generally found 

 in colonies, are sometimes solitary in their habits of life. One of the most nearly 



Fig. 108. — Fimgia symmetrica, three times the natural size. 



