50 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
peritoneal epithelium, which are set side by side in a single and very 
regular layer, are each prolonged into a delicate filament, which crosses 
the fibrillar layer perpendicularly, and passes on to be attached to the 
subjacent connective tissue. The histological constitution of this 
nervous layer is, therefore, identical with that of the ambulacral nervous 
system, save that the ectodermal cells of the latter are replaced in the 
former by enterocoelic cells. 
The author states that he has recognized this enterocoelic nervous 
system in all his types — Asterias glacialis and tenuispinis , Echinaster 
sepositus , and Astropecten aurantiacus. He has not been able to detect 
any communication between this centre and the intra-epithelial super- 
ficial plexus, which extends between the ectodermic cells of the outer 
wall of the body. 
He thinks that this new nervous system recalls in a singular manner 
that which is so well developed in Crinoids. If we make «■ transverse 
section of an arm of an Asterias or an Antedon we find exactly the same 
elements from the oral surface:— (1) Ectodermal nerve-band, con- 
tinuous with the epithelium of the ambulacra, and, in Asterias, with 
the ectodermal investment of the body ; (2) radial schizocoel- sinus, 
greatly developed in Asterias, more reduced, but certainly present in 
the Neocrinoidea ; (3) the radial ambulacral canal ; (4) a large cavity, 
the prolongation of the coelom of the disc, simple in Asteroids, divided 
by septa into three cavities in Crinoids ; (5) the aboral wall of the body, 
inclosing muscular fibres and nerve-cords, greatly developed in Crinoids. 
We know, moreover, that the axial nerves of Crinoids are of enterocoelic 
origin. The genital nerve-ring of Echinoids, discovered by Prouho in 
Echinus acutus and Strongylocentrotus lividus, and by the author in 
Arhacia pustulosa and Echinodiscus biforis, ought to be put in the same 
category. In Ophiurids the same part is represented by the aboral ring, 
which goes from the axial sinus to the genital organs ; this has been 
found in Ophiocoma scolopendrina, Opliiothrix fragilis, and others. It 
seems certain that the genital nerve-ring of Echinoidea and Ophiuroidea, 
like the aboral cords of Crinoidea and Asteroidea, are of mesodermal 
origin, and are developed at the expense of the enterocoelic epithelium ; 
this is one of the most interesting exceptions in the developmental 
history of the nerve-centres of Metazoa. No aboral nerve-centre has 
yet been seen in Synapta or Holothuria. 
Echinodermata of Yucatan and Vera Cruz.* — Mr. J. E. Ives 
describes the Echinoderms collected from these regions. Holothuria 
Heilprini sp. n. is related to H. atra , but the calcareous deposits are 
arranged in heaps, which produces an appearance of granulation over 
the surface of the body ; H. Silamensis is allied to the group which (in 
Thiel’s classification) contains H. marniorata, and while it presents 
differences from all members of it, it may, like them, or some of them, 
belong to one very variable and widely distributed species. H. nitida 
sp. n. is also allied to H. atra. The new genus Thyraster is instituted for 
Echinaster serpentarius, as the skeleton consists of quadrilateral plates 
arranged in regular longitudinal series, and not, as in Echinaster , of 
small narrow imbricated plates united into an irregular network. This 
* Proc. Acad. Nat. Sci. Philad., 1893, pp. 317-40 (1 pi.). 
