30 SIDERASTREA RADIANS. 



The free end of the mesenteries and, consequently, the filaments also are 

 somewhat exceptional among corals in the small degree to which they are 

 convoluted in the lower regions. In most cases they pass vertically in a 

 straight course from their upper to their lower extremities without any folding. 

 Upon irritation the living polyps of most corals are able to extrude, through 

 temporary apertures in the body-wall, masses of contorted filaments along 

 with the portion of the mesentery to which they are attached, but this 

 phenomenon has been observed on only one occasion in Siderastrea. 



The question of the ectodermal or endodermal origin of the filaments is 

 alluded to in the description of the larva. 



SKELETOTROPHIC TISSUES. 



The skeletotrophic or skeletogenic tissues constitute by far the greatest 

 proportion of the soft parts of the polyps, including as they do the lining of 

 the calicinal wall, septa, and columella throughout. They represent the 

 original flat basal disc of the polyp, which has become greatly infolded in 

 correspondence with the skeletal ingrowths. Both the endoderm and ecto- 

 derm present certain structural peculiarities compared with the same layers 

 in the column wall and disc, and in some respects the calicoblast layer of 

 S. radians differs from that in most other corals. 



The polypal wall lining the uppermost parts of the skeleton is extremely 

 delicate, especially over the edges of the septa. In sections the combined 

 ectoderm and endoderm vary from 0.015 to 0.003 mm - ^ n thickness, both layers 

 being about equal (plate 8, fig. 47). The endoderm is a syncytium showing 

 no signs of cellular divisions, and the cytoplasm has at first few or no 

 vacuoles, though below they become somewhat numerous. The contained 

 nuclei are large, round or oval, and closely arranged in a very regular row. 

 Zooxanthellse also occur, their diameter being often equal to the thickness 

 of the layer. Irregular mucous spaces, similar to those described in the 

 endoderm (p. 22), are also seen, except where the layer is at its thinnest. 



The mesogloea, both here and elsewhere throughout the skeletogenic 

 tissues, is rarely distinguishable as a distinct layer, but constitutes a mere 

 line of division between the ectoderm and endoderm. Where the mesogloea of 

 the mesenteries is united with the skeletal covering the calicoblast ectoderm 

 is nearly absent, and the mesogloea may then become swollen and continued 

 into the well-known structures which have been termed desmocytes by 

 Bourne (1899). The combined mesenterial and skeletotrophic mesogloea 

 here broadens in a fan-like manner, is striated, and stains more deeply than 

 usual (plate 8, fig. 45). Sometimes the desmocytes appear as more distinct 



