SOLITARY CORALS. 223 



ectoderm. In Heteropsammia these canals form part of the system of endodermic 

 canals characteristic of perforate corals, but their presence in Heterocyathus is 

 remarkable. The transverse section has a curious resemblance to a section through a 

 young Actinian with twelve mesenteries and a very wide stomodseum. The ectoderm 

 lining the central tube is curiously modified at its inner end, as will be described 

 below. The tubes vary in number and position. There may be from five to nine of 

 them, and they are not, as a rule, in the same plane, but in the majority of specimens 

 they are more numerous and more closely set together on the side furthest from the 

 mouth of the Aspidosiphon chamber. Tangential sections of the corallum show that 

 the tubes are interseptal, and that the stereoplasm filling up the interseptal loculi is 

 interrupted by their presence. Being interseptal, their external openings are always 

 between the costse. 



Histology. Though Professor Herdman's specimens are exceptionally well 

 preserved, histological details are, as is usual in corals, difficult to determine to one's 

 satisfaction. In what follows I do not profess to give a complete account of the 

 histology of the different tissues, but will confine myself to such details as I have 

 been able to make out to my own satisfaction. 



The E c t o d e r m . As is shown in fig. III., the ectoderm of the body-wall is more 

 or less deeply infolded between the costre in spirit specimens. It is thinner where it 

 is stretched over the edges of the costse and thicker in the furrows between, and this 

 does not seem to be due to contraction in spirit and the consequent stretching of the 

 tissue over the costal edges, but to a differentiation of the ectoderm, which is not 

 only thicker, but more glandular and more richly provided with nematocysts in 

 longitudinal stripes, corresponding to the attachments of the mesenteries. In the 

 thinner stripes of ectoderm corresponding to the costse the tissue consists almost 

 entirely of columnar or cubical epithelial cells. There are very few gland cells, and 

 few, if any, nematocysts. In the thicker stripes opposite the attachments of the 

 mesenteries the epithelial cells are longer, and there are numerous gland cells and 

 nematocysts of the kind shown in Plate III., fig. 18. The gland cells are of an 

 elongated goblet form, with a comjjressed nucleus at the base of the goblet and a, 

 very thin protoplasmic stalk passing from the nucleus to the mesogloea. They 

 contain a number of yellowish-brown highly refractive granules, which do not stain 

 with any of the ordinary aniline dyes, or with hematoxylin. Similar gland cells are 

 very abundant in the ectoderm of the tentacles. The nematocysts, two of which are 

 shown everted in fig. 16, are elongate oval in shape with a somewhat coarse thread 

 coiled loosely within. The everted threads are covered with long barbs disjiosed in a 

 spiral. Before eversion these nematocysts contain a flocculent substance which stains 

 bright blue in picro-indigo-carmine, a7id therefore probably belongs to the class of 

 hyalogens, all of which stain similarly with this dye. 



The ectoderm lining the outer moieties of the tubes leading into the Aspidosiphon 

 chamber is of the same character as that of the thickened stripes of the body-wall, 



