460 TAHITI. 



present polyps which have become solely reproductive in 

 function, just as the dactylozooids have become solely tenta- 

 cular in function. Hence, in these colonies certain members 

 of the community devote themselves to the catching of food, 

 but cannot eat it themselves ; 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 ovvn. Other members again 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 Sfyhisferidce, the polyps are lodged within pores of 

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

 the dactylopores are far more numerous than the gastropores. 

 In some genera of StylasteridcB, the pores are scattered irre- 

 gularly 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 i represents the con- 

 dition existing in the genus Sporadopora, 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 Stylasteridie 

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

 and which by its presence gives the name to the family, 

 StyJasleridcB. In Sporadopora. the pores of the two kinds are 

 irregularly scattered over the whole coral surface. 



In the case of another Sty/asterid, AUopora nobi/is, 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 gastropore, just as in the Tahitian Miliepora 

 (see Fig. 2). In other parts of the same specimen, a further 

 complication arises, as shown in Fig. 3. A shallow groove 

 leads from each of the dactylopores to join the gastropore 

 cavity, and a radiate figure is produced. No doubt the 

 grooves are developed by the constant bending inwards of the 



