///. ANTIIOZOA. 



25.5 



and have iissumed a plan of four or ten, but without altering the primitive 

 conditions. 



In the Octocoralla (fig. 204) tlie conditions 

 are simpler, only eight septa being developed. 

 These are disposed equally on either side of the 

 ojsophagus and may have (most octocorallans) 

 all their muscles towards one end, or (Edwardsia, 

 fig. 30.5, IV) may have the muscles of one pair 

 reversed. It is to be noted that hexactinians 

 pass through an Edwardsia stage. In Cerianthus 

 new septa are always added at one end of the sag- j-,,,. 204.-Transverse sec- 

 ittal axis (tig 305, II), while in the extinct Tetra- t'on of an Octooorallan 

 coralla (fig. 305, I), so far as one may judge from Ke'l't se^tToi^Zl 

 the hard parts, the septa liavo an arrangement 'StonTSk.'}^LS^^l2 

 with four as tljc basis. with those of the other 



side. 



I'lG. 20.5.— Arrangement of septa in various Aftinozoa. I, Tetracoralhi ; 11, Cerian- 

 thus; III, Ootuooralla; IV, Edwardsia. 



Bj' far the greater jDart of tlie Aiithozoa rei^roduce hj buddino- 

 lis well as bjf eggs. Only rarely do the buds separate, but geiierallv 

 they remain connected with the mother to form a colony of hun- 

 dreds or thousands of individuals. These are connected by an 

 extensive cwnencliym or crenosarc, consisting largely of mesoglo^a, 

 but having an outer coat of ectoderm and penetrated by a system 

 of branching and anastomosing entodermal canals (fig. 20G). On 

 disturbance the polyps can quickly retract themselves into the 

 ctt'nosarc. 



The colonial Anthozoa have almost invariably a skeleton, 

 secreted by the ectoderm and consisting either of calcic carbonate 

 or of an organic horn-like substancj. Sometimes the horn and 

 lime alternate. One recognizes an axial and a cortical substance. 

 The axial skeleton is confined to the deeper iDortions of the cceuosarc, 

 while the cortical jjortions are formed liy the polyps themselves 

 anil to a large extent (figs. 207, 298) repeat their complicated 

 structnre. Except in a few forms [Fiiiigia) a theca is present; 

 this is a calcareons cup, and from this usually extend inward 

 calcareous partitions called, in distinction to the fieshy- or sarco- 

 septa, the sclerosepta. 



