ACTINOZOA. 153 



is due to the presence of pigment-streaks or less 

 regular stellate masses, in various parts of the 

 ectoderm. 



6. Corallum or l§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 known 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 



