530 A NATURALIST ON THE "CHALLENGER." 



sent polyps which have become solely reproductive in function, 

 just as the dactylozooids have become solely tentacular in function. 

 Hence, in these colonies certain members of the community 

 devote themselves to the catching of food, but cannot eat it them- 

 selves ; they deliver it to other members of the colony, whose only 

 function is to eat and digest it. These latter nourish the whole 

 colony by supplying blood to it through the common circulation 

 as the products of their digestion ; in several genera, they have 

 become reduced to the condition of mere stomachs, having no 

 tentacles or prehensile organs of their own. Other members 

 ao'ain of the colonies neither catch food nor eat it, but are 

 entirely devoted to the production of eggs and larvae, and have 

 thus become reduced to the condition of mere egg-bags. 



In the Stylasteridce, the polyps are lodged within pores of 

 two kinds, just as in the Milleporidw, and, as in these latter, the 

 dactylopores are far more numerous than the gastropores. In 

 some genera of Stylasteridce, the pores are scattered irregularly 

 all over the surfaces of the coral stocks ; but in others they are 

 grouped into systems of very great complexity, and almost all 

 gradations of this complexity are shown in the various genera, so 

 that the successive stages by which natural selection has brought 

 about the development of the systems is clearly to be traced. 



This series of stages of development is shown in the set of 

 diagrams on the opposite page. Figure 1 represents the con- 

 dition existing in the genus Sjporadopora, the dactylopores shown 

 as the smaller black circles are here irregularly grouped together 

 with a single large gastropore. The gastropore has a white 

 dot in its centre, marked S, indicating the " style," a rod 

 of the calcareous skeleton, which in many genera of Stylasteridce 

 acts as a support to the mouth-bearing polyp within its pore and 

 which by its presence gives the name to the family, Stylasteridce. 

 In Sporadopora, the pores of the two kinds are irregularly 

 scattered over the whole coral surface. 



In the case of another Stylasterid, Allopora nobilis, the 

 development of regular systems of polyps is commenced. The 

 arrangement is shown in Figs. 2 and 3. In some parts of the 

 branches of a specimen of this Coral, the dactylopores are to be 

 found simply grouped in rings around a single centrally-placed 



