TAHITI. 



533 



out by mutual appressure. The elongate or groove-like form of 

 the mouths of the pores is also to a large extent brought about 

 by the manner in which the dactylozooids are doubled up within 

 them when in the retracted condition. 



The figure on the opposite page represents a small part of 

 the skeleton or coral of a stock of Astylus subviridis, enlarged to 

 twice the natural size. The cyclosystems, one of which is 

 shown as a diagram in Fig. 6 of the preceding woodcut, are here 

 seen placed at intervals along the branches of the coral. 



Still further complexity, however, in the cyclosystems of the 

 genus Astylus remains to be described. The figure below shows 

 one of the systems cut through vertically to display the arrange- 

 ments within. The gastropore has two chambers, an upper and 

 lower. The lower, in which the gastrozooid, which in this genus 

 is a mere flask-shaped stomach sac devoid of tentacles, is lodged, 

 communicates with the upper by a narrowed horseshoe-shaped 

 opening, which is more plainly seen from above in the diagram, 

 Fig. 6, already referred to. The opening is rendered horseshoe- 

 shaped by the projection from one side of it across the aperture 

 of a small tongue-shaped excrescence of hard coral. This pro- 

 jection no doubt serves to protect the polyp from injury. 



VERTICAL SECTION THROUGH ONE OF THE CYCLOSYSTEMS OF ASTYLUS SUBVIRIDIS. 



dp-dp Dactylopore cavities; gp upper gastropore chamber; gp^ lower gastropore chamber; 

 cc canals leading from the gastropore to the dactylopore cavitie9 ; b tongue-shaped projection ; 

 a its base cut through ; gg ampullas cut open. 



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 



