ADULT COLONY. 55 



DISSEPIMENTS. 



The dissepiments are extremely thin and delicate transverse partitions 

 which serve to cut off the living polyps from the dead part of the skeleton 

 below. In section they are only o.oi rnm. thick. Along with the columella 

 they serve for the time being as a support for the basal part of the polyp, the 

 dissepiment being morphologically comparable with the basal plate of the 

 larval polyp. In some cases the partitions are horizontal, but in others they 

 are deeply convex upwardly. They are situated at somewhat different levels 

 in different interseptal loculi, and in the same loculus vary from 0.25 mm. to 

 0.5 mm. apart. The distance apart of two adjacent dissepiments in a loculus 

 represents a period of growth of the polyp in its march upward, the polyps 

 throughout their lifetime retaining approximately the same vertical length 

 and transverse diameter. As a result of the very narrow interseptal loculi in 

 Siderastrea the dissepiments are rather insignificant features of the corallum, 

 but as a basal support they are probably as important to the polyp as in other 

 species of corals in which they are more prominent. Occasionally a dissepi- 

 ment is found intersected by a synapticulum, when both n^-be considered as 

 constituting the basal support of the particular lamella. Such was probably 

 the case in the interseptal lamella represented in plate 6, fig. 33. Usually 

 the lamellae present a more nearly straight basal extremity than is here repre- 

 sented, showing that the polyp is cut off below in a regular manner by the 

 formation of dissepiments which alternate with the synapticula. The dissepi- 

 ments may thus correspond or alternate with the synapticula, the latter being 

 formed independently and in advance of them. 



The dissepiments are easily studied in tangential sections of corallites, 

 and it is found that their histological structure is different from that of 

 septa. They possess no centers of calcification, but are thin and laminated, 

 composed of fibro-crystals standing at right angles to the surface. They thus 

 recall the microscopic structure of the basal plate and epitheca, and like these 

 they are lined by the calicoblasts of the polyp only on one side. 



Miss Ogilvie (1897, p. 178) assumes that the synapticula and not 

 the dissepiments constitute the principal basal support of the polyp in 

 Siderastrea. This would follow were the synapticula formed, as she assumes, 

 within special interseptal invaginations of the aboral body wall ; but, as 

 shown on p. 53, such is not their origin. They are produced by special areas 

 of the septal iuvagination, ultimately resulting in perforation of the polypal 

 wall. As the synapticula actually perforate the polyp they may be conceived 

 as also affording it support ; but only incidentally, as happens to be the case 



