%9y!'''] PROCEEDINGS OF THE NATIONAL ftlUSEUM. 143 



in about three cycles. They are .simple, cylindrical and taper to a point, 

 and cover almost the entire disk. There being no special sphincter 

 muscle, the tentacles are not covered in contraction. 



There is no conchula, and only one siphonoglyphe, which is neither 

 very deei) nor well defined. The surface of the stomatoda'um ))ossesses 

 numerous ridges, which are high (PI. xxi, Fig. 1-4, st.) and may bifurcate 

 at the extremity or give off' secondary ridges. They are more uumer 

 ous than the mesenteries, and «lo not seem to stand in any very 

 delinite relation to them. The juesenteries are twenty iu number. 

 They are arranged in pairs, two of the pairs being directives, and are 

 all perfect. Below, however, it is seen tliat four of the pairs, as in 

 Pearhid, are much narrower than tlie other six, these narrow ])airs being 

 situated in the sulco lateral and lateral intermesenterial si)aces. The 

 mesenteries are thin ; at the base there are jtinnately arranged mtiscle 

 I)rocesses (PI. xx, Fig. !.'{, btu.), and se])arated from these by a. region in 

 which the mesenterj'^ is exceedingly thin are the longitudinal mus(;les. 

 These are very strong (PI. xx. Fig. l.'J), but at the same time narrow, 

 forming a strong ])i-otuberance upon the surface of the mesentery. 

 Above, however, they a\ iden out (PI. xxi. Fig. 14) and the jaocesses are 

 not so high. 



All the mesenteries bear reproductive organs. * 



There are a. few points in the histology of this species which are inter- 

 esting. The mesoglani is fibrillar, especially towards its inner surfac(% 

 and contains very numerous cells. It is in the ectoderm however, tliat 

 the most interesting peculiarities ap])ear. Tiie ectoderm of tlie column 

 wall is high and contains, as usual, many gland cells. In addition to the 

 usual elements, however, it also contains numerous nematocysts (PI. xxi, 

 Fig. 15, ;/) lying in its outer portion, .sometimes very closely crowded 

 together. Immediately external to and resting upon the mesogkea, 

 roundish bodies— or, rather, bodies appearing round in cross-section 

 (mf.) — which stain somewhat deeply, can be i>erceived. These seem to 

 be muscle fibres, having a longitudinal direction. They have all the 

 appearance of muscle fibres, but I was not able to render their nature 

 certain by the study of maceration preparations. Futlier evidence for 

 their muscular nature is, however, to be found in tlie pj'esence, exte- 

 rior to them, of a thin layer of tibrilhe having all the appearance of a 

 nerve layer. 



Longitudinal muscles and a nerve layer are, as a rule, absent in the col- 

 umn wall of the Hexactiniae; but, on the other hand, are well developed 

 in the CerianthejT', and it seems probable that the more i>rimitive Actino- 

 zoa likewise i)ossessed them. Hitherto tliey liave been found among 

 the Hexactiniae only in Corynactisf sp'? and GorallimorphKs obtecti(,s,\n 

 which forms they have been described by Hertwig ('88). The fibres 

 ofHalcurias resemble those of Cor<(llimorphus in being poorly developed, 

 and are apparently fewer in number. In ('orf/nactis? on the other 

 liaiul, they seem to reach a fair degree of development. 



