230 



CCELENTERATA 



Order I. Tetracoralla (Rugosa). 



Extinct forms from the paleozoic rocks with the parts arranged in fours (fig. 

 196, I). The present tendency is to regard them as modified Hexacoralla. 



Order II. Octocoralla (Alcyonaria) . 



These forms, which have eight single septa, are recognizable by their eight 

 feathered tentacles (fig. 197). They occur in all seas from near the shore to great 

 depths. In development there is a planula (fig. 201) in which the oesophagus 

 arises as a solid ingrowth which becomes perforated later. The eight septa 

 arise simultaneously. Usually colonies are formed by budding and a poly- 

 morphism may occur, some individuals which have reduced septa and lack 

 tentacles, taking in water for the colony. Many are phosphorescent. 



B 





V 



FIG. 201. FIG. 202. 



FIG. 201. Three stages in development of Renilla reniformis (after Wilson). 

 A, cleavage of egg; B, planula; C, development of oesophagus; ec, ectoderm; en, ento- 

 derm; ;, mesoglcea; o, oesophagus. 



FIG. 202. American sea-anemones. A, Edward sieUa sipunculoides (after Stimp- 

 son). B, Bicidium parasiticum (after Verrill) . C, Bunodes stella (after Verrill). 



ALCYOXIID^; (Alcyonium*') , axial skeleton is lacking, the flesh contains 

 numerous calcareous particles (sclerodcrmitcs). The sea pens, PENNATULID^:, 

 have the basal part buried in the mud, the rest, expanded like a disc or feather, 

 bears the polyps. An axial skeleton usually occurs in the stalk. Peimatnla* 

 Renilla*. The GORGONIID.E (sea fans, sea whips) have an axis of more firm- 

 ness, which may be calcareous, and the colony may branch and the branches 

 anastomose. Here belongs, besides many tropical genera whose names end in 

 'gorgia,' the precious coral (Corallium rubrum, fig. 197), the fishing for which at 

 Naples amounts yearly to half a million dollars. TUBIPORDXE, organ-pipe 

 corals. The HELIOPORHXE were long regarded as Hexacoralla because of their 

 massive skeletons with six sclerosepta. The paleozoic Syringopora belongs 

 near Tubipora, while the FAVOSITID^E resemble the Alcyoniidae. 



Order III. Hexacoralla (Zoantharia). 



The simple tubular tentacles are highly characteristic of the Hexacoralla, 

 as is the arrangement of the paired septa in sixes as described above. Yet there 

 are exceptions to this rule. On the one hand is Edwardsia* with sixteen or 



