ACTINOZOA. 153 



is due to the presence of pigment-streaks or less 

 regular stellate masses, in various parts of the 

 ectoderm. 



6. Coralliim or S»keleton. — Intimately con- 

 nected with the tegumentary organs of these 

 animals, under which head, indeed, it might with- 

 out impropriety be described, is the so-called 

 skeleton, or ' corallum ', with which so many of 

 them are furnished. 



The term coral, or corallum, is properly re- 

 stricted, in zoology, to the hard structures deposited 

 in the tissues, or by the tissues, of the Actinozoa. 

 Any form of this class which possesses such a 

 framework is called a ' Coral '. 



All Actinozoa are not coralligenous. The Cteno- 

 phora and several species of Zoantharia deposit 

 no corallum. On the other hand, the order 

 Rugosa is kno^vn only from the remains of extinct 

 Corals. 



Of coral structures there are two principal 

 kinds, which must be carefully distinguished from 

 one another. First, the ' sclerobasic ' corallum, 

 a true tegumentary excretion, formed by the 

 conversion of successive growths from the outer 

 surface of the ecderon. Secondly, the * sclero- 

 dermic ' corallum, which better merits the name 

 of skeleton, deposited, as it is, within the tissues 

 of the animal, and, in all probability, by the en- 

 deron. 



The sclerobasic corallum is by Mr. Dana termed 

 " foot secretion " ; the sclerodermic, " tissue se- 

 cretion ". 



Let us first notice the sclerobasic corallum, 

 which is found only in certain budding composite 



