122 SIDERASTREA RADIANS. 



YOUNG POLYPS. 



Very few of the young polyps reared were available for anatomical and 

 histological study, owing to the greater importance of the corallum at this 

 stage ; in most cases the soft tissues were removed by maceration in order to 

 secure the skeleton. A vertical section through a decalcified retracted larval 

 polyp, of about two months, is represented on plate 9, fig. 53. The section 

 of the skeleton has also been added, the details being taken from various 

 coralla of this age. The section of the polyps includes the oral aperture, and 

 on the right is truly radial, passing through a mesenterial space and cutting 

 a septal invagination obliquely, while on the left side it passes obliquely 

 through a mesentery and also through a septal invagination. A third invag- 

 ination is included in the middle region of the section and probably represents 

 an early columellar upgrowth. The upper wall on the left has come to rest 

 upon one of the invaginations. 



The septal and columellar invaginations are here of the simplest charac- 

 ter. They are merely continuous upgrowths of the basal wall of the polyp, 

 and agree with it histologically. They line both sides of the calcareous 

 upgrowths from the basal plate, while the latter in its turn is laid down by 

 the flattened part of the disc, increase in thickness taking place only on 

 the upper side. 



Sections of coral polyps at such an early stage are the most favorable for 

 demonstrating conclusively that the madreporarian skeleton is laid down 

 wholly outside the polypal tissues, and that all the skeletal complications are 

 formed within extensions of the basal disc, the invaginations proceeding pari 

 passu with the deposition of calcareous matter. This truth was first estab- 

 lished by von Koch by means of sections of polyps of Aslroides, somewhat 

 similar to that of plate 9, fig. 53 ; further, von Koch was able to demonstrate 

 the calcareous spheroids of the basal plate and septa in situ. His polyps 

 for this purpose were adherent to pieces of cork, so that the polyp and its 

 attachment could be sectionized together. The polyps of Siderastrea being 

 adherent to glass could be sectionized only after being freed through decal- 

 cification. 



Decalcification of the polyp from which fig. 53 was taken was carried out 

 with great care, but only fragments of a very narrow skeletogenic ectoderm 

 remained. No hints of any mesoglceal processes or desmocytes occurred, 

 but such would scarcely be expected considering their rarity in the adult 

 polyp of Siderastrea. The mesogloea itself is a very thin lamella. The basal 



