ADULT COLONY. 23 



spaces has been observed in the Zoantharia. Its appearance in sections of 

 Siderastrea stained in haematoxylin is very characteristic, and its presence 

 should be looked for in other forms. 



TENTACLES. 



When the polyps are preserved in the retracted condition, with the 

 tentacles sessile, the columnar and discal walls come to lie upon the skele- 

 totrophic layer of the septal edges, and the two knobs of the bifurcated 

 tentacles are far apart, one on each side of a septal ridge (plate 7, fig. 40), 

 while the knob of the simple tentacles lies immediately over the septum with 

 which it corresponds. In this condition the stem of the tentacle has altogether 

 disappeared as such, having become part of the disc. As a result the tenta- 

 cles are recognizable in vertical sections of polyps as mere hemispheroidal 

 thickenings of the ectoderm. In the case of the eutoccelic tentacles a thick- 

 ening occurs on each side of a septal elevation of the disc, while a single 

 elevation directly over the apex of the septum represents an exotentacle. 

 In the thickenings long nematocysts extend nearly across the ectoderm, and 

 smaller ones crowd the periphery. The two forms of nematocysts are 

 represented on plate 7, fig. 44. Histologically the portion of the wall join- 

 ing the two tentacular knobs on plate 7, fig. 40, and representing the stem of 

 the tentacle, differs in no way from that of the disc. 



In sections of polyps preserved with the tentacles extended the organs 

 appear as tubular outgrowths of the disc, terminated by knoblike enlarge- 

 ments. The stem seems to differ in no respect histologically from the disc, 

 not even in the greater number of nematocysts ; the apex alone constitutes a 

 battery of stinging cells. The cut ends of a delicate ectodermal muscle 

 layer can be distinguished in transverse sections of the stems, and a feeble 

 circular endodermal layer in longitudinal sections. The tentacular muscu- 

 lature is continuous with that of the disc, and a very definite nerve layer 

 occurs at the apex of the tentacles (plate 8, fig. 49). 



STOMOD^EUM. 



The stomodseum is a short, thin-walled, depending tube, strongly ciliated 

 all round, and without any permanent ridges or grooves. Usually it is 

 oval in transverse section, and along its endodermal surface six pairs of 

 mesenteries are attached at equal distances apart. Histologically it is of 

 the same structure all the way round, there being no modification opposite 

 the directive mesenteries such as can be regarded as a siphonoglyph or 

 gonidial groove. This is also the case in all madreporarian polyps yet 

 described. 



