534 A NATURALIST ON THE " CHALLENGER." 



dactylozooids and nourish them, The slit-like openings of the 

 dactylopores into the upper chamber of the gastropore, allow 

 the dactylozooids to be bent far down into the gastropore to 

 reach the gastrozooid, and deliver food to it. 



Around each cyclosystem are grouped a zone of ampulla?, 

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

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

 shown cut open in the figure, and marked GG. Thus each cyclo- 

 system 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 numerous 

 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, Disticlwpora (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 Milleporidce and Stylasteridce are so closely allied to one 

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

 Hydroid corals (Hydrocorallince). The Milleporidce all occur in 

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

 teridw, on the other hand, although some species occur in quite 

 shallow water on reefs, range also to great depths, some species 

 having been dredged by the "Challenger" from 1,500 fathoms. 

 Specimens of six genera of Stylasteridce were fished up at one 

 haul of the trawl, from 600 fathoms, off the mouth of the Rio de 

 la Plata, and it was from these specimens that the details of the 

 structure of the family were made out, and the Stylasteridw 

 determined to be Hydrozoa. 



