112 SIDERASTREA RADIANS. 



In Astroides calycularis Von Koch (1882) found the basal plate to take 

 the form of a thin circular disc, composed of spheroidal or elliptical crystal- 

 line bodies, from 0.005 to 0.008 mm. in transverse section. At first the 

 calcareous elements were arranged in an interrupted manner, but later the 

 openings were filled by further deposit of skeletal matter, and the plate as a 

 whole became thickened. In Caryophyllia cyathus (1897) the first rudiments 

 of the basal plate consisted of a small central deposit surrounded by six thin, 

 nearly triangular plates, interseptal in position. For a time these were dis- 

 tinct from one another, but later united with the central circular patch, and, 

 by further additions, became joined along their edges, thus constituting a 

 complete flattened disc or plate. The presence of a basal plate has since 

 been recognized in many forms of corals, and it is extremely doubtful 

 whether it is really wanting in any species. 



For a long time the developing polyps of S. radians gave no external 

 indication of any skeletal formation which could be regarded as the 

 basal plate. The first evidences of the corallum were the six entoccelic 

 septa, which appeared to rise directly from the surface of attachment. 

 Under transmitted light they stood out as nearly opaque objects, while the 

 interseptal spaces at this and later stages were quite clear and apparently 

 devoid of any calcareous crystals. For nearly two months the laboratory 

 notes contained the assertion that no basal plate was developed in Siderastrea, 

 for, excepting the presence of the septa and epitheca, there was no interrup- 

 tion in the ordinary light passing through the polyp. When, however, the 

 living or preserved polyps were examined by means of polarized light, the 

 bright colors of the basal region revealed the presence of crystalline matter. 

 This is well seen in the photograph reproduced on plate 4, fig. 22, taken 

 with an exposure of twenty minutes to polarized light. Between crossed 

 nicols the field was black except in the region of the polyp ; the septa also 

 appeared black owing to their thickness and the irregular disposition of their 

 crystalline constituents, while all the interseptal areas were brightly and 

 variously colored. 



It is obvious, therefore, that a basal plate was already developed between 

 the polyp and its surface of attachment, but was too thin to offer any appre- 

 ciable obstruction to the transmission of ordinary light. The plate must have 

 begun to form a day or two after fixation of the larva, for the septal upgrowths 

 were observed on the third and fourth days, and the deposit of basal skeletal 

 matter probably preceded them. As stated below, the plate does not undergo 

 much thickness, even in later stages ; hence, at no time would it offer much 

 interference to the passage of light 



