CORALS. 



59 



seemed to become exhausted, contracted, and became attached. The transition 

 from the worm-like larva to the polyp takes place as in the anemones. The 

 thicker end of the body is pressed against a hard rock, and the whole contracts 

 into a thick, round disc ; while longitudinal furrows become visible at the upper 

 pole, where the mouth sinks deeper. At the ends of these furrows the twelve 

 tentacles appear. The accompanying three illustrations show the stages which 



DEVELOPMENTAL STAGES OF Astroides catycularis (magnified 24 times). 



follow in rapid succession, resulting in a form almost exactly like a young sea- 

 anemone. It has, however, already commenced to secrete its calcareous skeleton. 

 This is not formed as a connected whole, but begins as a number of separate centres 

 of secretion between the polyp and the substance to which it is fixed. These meet 

 and fuse, till gradually the skeleton is produced. The polyp commences to bud, 

 and the buds develop their skeletons, the whole together forming a stock like that 

 shown in the illustration. 



The star-corals, which are some of the principal reef -builders, do not branch, 

 but form great solid mounds; 

 the polyps being all cemented 

 together, and the budding so 

 arranged that the whole colony 

 forms a thin, living layer or 

 covering to the mass it and its 

 parents have built up; all but 

 this thin layer on the surface 

 being dead coral. The illustra- 

 tion given is of Astrcea pallida, 

 a species which appears as a 

 rounded mass, with flat base, and 

 the individuals being quite dis- 

 tinct from one another, although their outer walls are in contact. Those on 

 the top and to the right of the figure are represented in a contracted condition, 

 and the rest with expanded tentacles. None of the individuals here seen are in 

 the act of dividing; and the genus is characterised by the fact that the bud- 



A STAR-CORAL, Astrcea (I nat. size). 



