voL.xvij PROCEEDINGS OF THE NATIONAL MUSEUM. 177 



1893. 



consistency of the jnesogkea. The sphincter muscle (PI. xxviii, Fig. 

 73) is fairly strong and in its upper part occupies nearly the whole thick- 

 ness of themesoglcea, being sei)arated from the ectoderm and endoderm 

 by thin laj ers of mesogkiea. It is composed of very numerous more or 

 less circular (in section) cavities lined with muscle cells, and so closely 

 arranged as to be separated only by very narrow bands of fibrous me- 

 soglcea (PI. XXIX, Fig. 7G). In consequence of their arrangement this 

 portion of the column wall, under low magnification seems to have a 

 reticular structure. The sphincter extends a considerable distance 

 down the column, becoming thinner and having the cavities more sep- 

 arated in its lower part, until finally they are scattered singly or in 

 pairs in the lowermost portions. 



The tentacles are slender and acuminate, and their number I estimate 

 at slightly below one hundred, though I was unable to make a definite 

 count. They have a cream-white color. Their longitudinal muscula- 

 ture is ectodermal, and the mesogiceal supporting processes are fairly 

 strong. Large numbers of nematocysts occur in their ectoderm. 



The disc has strong radiating ridges corresponding to the endoccels 

 of the first and second cycles of mesenteries, and has its radiating 

 musculature ectodermal, like the longitudinal muscles of the tentacles. 

 En the ectoderm of the disk are numerous oval or spherical bodies, of 

 a granular structure, which stain deeply Avitli borax carmine. I could 

 not detect a nucleus in any of them. Their abundance and general ap- 

 pearance seem to preclude the idea that they are foreign bodies, and 

 the only explanation as to their significance which suggests itself is 

 that they are glandular bodies. The preservation of the ectoderm was 

 not sufficiently perfect, however, to allow of any certainty on this point. 



The stomatod;eum is rather small in diameter, and possesses about 

 ten longitudinal ridges; in some specimens there was only a single 

 siphonoglyphe, but Avhether this is a characteristic arrangement I can 

 not say. Judging from the observations of G. F. and A. Y. Dixon 

 on various species of Sagartia ('88) and my own ('0 1 ) on Metridimn 

 marginatum, it is more probable that there is a variation in the number 

 of siphonoglyidies, some specimens i^ossessing only one and others two. 

 As in the case of Mctridiuni and Sagartia vennsta, there is only one pair 

 of directives in tliose specimens of S. lactca Avhicli possess a single 

 siphonoglyphe. 



The mesenteries are arranged upon the decamerous plan, there being 

 in all ten pairs of perfect mesenteries, all o^' which, with the exception 

 of the directives, are gonophoric. 1 was in hopes that it might be possi- 

 ble, from the distribution of the reproductive organs upon the mesen- 

 teries, to ascertain which of the mesenteries of the second cycle it was 

 which had failed to develop, the normal hexamerous arrangement 

 being thus converted into a decamerous one; but in this I was disap- 

 pointed. Counting the ten perfect pairs of mesenteries as representing 

 two cycles, one of which, the second, is not <|uite completCj there is present 

 Proc, N. M. 93 12 



