470 STRUCTURE OF CORAL. 



in size, as soon as they become distinct. In germination, 

 on the other hand, the polype-bud consists at first of 

 nothing more than an outer and inner skin, ectoderm and 

 entoderm, enclosing a coecal process of the body-cavity, and 

 possessing neither mouth nor other structures. 



The coral structures resulting from the fissiparous mode 

 of reproduction are of two kinds, according as they tend 

 to increase in a vertical or in a horizontal direction. 



In the former case, the corallum, or calcareous skeleton, 

 the bone or internal framework of the animal, is cespitose, 

 or tufted, and may be resolved into a succession of short 

 diverging pairs of branches, each the result of the self- 

 division of a single corallite. 



In the latter case, the coraUum, or calcareous skeleton, 

 becomes lamellar. Here the secondary corallites are 

 united throughout their whole height, and being disposed 

 in a linear series, the whole mass forms one continuous 

 sheath or receptacle (tkeca). 



But both kinds of corallum are frequently made mas- 

 sive by the union of several rows or tufts of corallites 

 throughout the whole or a portion of their height. Pro- 

 fessor Greene refers, as an illustration, to the large gyrate 

 or convoluted corallum of the Meandrina, over the sur- 

 face of whose spheroidal mass the combined corallites are 

 wound and involved in a manner so complex as at once to 

 suggest the resemblance to the convolutions of the human 

 brain, which its popular name of brainstone coral indicates. 



Many coral -producing organisms are also included in 

 the Gorgonidce, or " sea-shrubs," the Porites, the Madre- 

 poridce, and the Milleporidce. 



To the labours of these animals we owe the formation 



