Chap. XX.] STRUCTURE OF THE STYLASTERID.K. 463 



The lower gastropore chamber further communicates, as 

 seen in the figure, at its margins by means of vertical canals 

 with the bottoms of the dactylopores, which are seen above 

 It. Through these vertical canals in the living coral pass large 

 nutritive vessels from the stomach of the gastrozooid all round 

 to join the dactylozooids and nourish them. The slit-like 

 openings of the dactylopores into the upper chamber (jf the 

 gastropore, allow the dactylozooids to b-e bent far down 

 into the gastropore to reach the gastrozooid, and deliver 

 food to it. 



Around each cyclosystem are grouped a zone of ampulte, 

 which contain the reproductive elements, and in which, in the 

 case of female colonies, the young are developed. The ampiilhe 

 are shown cut open in the figure, 

 and marked GG. Thus each 

 cyclosystem is, in the genus 

 Astylus, complete in itself. It 

 contains its single gastropore, 

 which placed in the centre 

 nourishes the whole, its zone of 

 dactylopores, and its zone of nurse 

 structures which produce and rear 

 its young. Nevertheless, the nu- 

 merous cyclosystems of the colony 

 are in communication with one 

 another by a common canal 

 system traversing its branches, 

 and thus each is able to assist 

 the other with nourishment, and 

 any part of the branches thus so 

 perfectly fed is able to increase 

 the size of colony, by growing, 

 and developing on the new twigs fresh cyclosystems as buds. In 

 some other genera of Stylasteridce, various other complications 

 in the grouping and structure of the pores and their zooids 

 occur. In one genus, Distichopora (see Diagram 7, ante), the 

 gastropores are arranged in regular rows at the edges of the 

 coral branches, and on either side of the row of gastropores is 

 placed a row of dactylopores. The pores are thus everywhere 

 grouped in three parallel rows, and they occur only on the 

 edges of the branches \ the rest of the coral surface is devoid 

 of pores altogether. 



The Milleporidie and Sty las ten dee are so closely allied to one 

 another that I have grouped them together as a sub-order to 

 Hydroid corals {Hydrocomllimc). The Milleponda all occur 

 in comparatively shallow water, and are reef corals. The Stylas 



VERTICAL SECTION THROUGH 



ONE OF THE CYCLOSYSTEMS OF 

 ASTYLUS SUBVIRIDIS. 



dp-dp Dactylopore cavities ; gp upper 

 g;astropore chamber ; tjij lower 

 gastropore chamber ; cc canals 

 leading: from the gastropore to 

 the dactylopore cavities ; 6 tongue- 

 shaped projection ; a its base cut 

 through ; yg ampullae cut open. 



