CRINOIDS. 183 
t relict of an earlier period of development. In the Ophi- 
irans the oral canal opens directly into the body-cavity ; 
n Echinothriz directly connects with the outer world by 
neans of the interradial canals. Finally, he regards the 
nervous vessel as homologous with the ventral vessel of the 
worms. 
Having made ourselves acquainted with the general struc- 
ture of the Echinoderms as exemplified in the star-fish, we 
are prepared to study the modifications of the Echinoderm 
plan in the different classes. 
Crass I.—Crinorpza (Stone-lilies, Encrinites, etc.) 
Order 1. Brachiata.—The living representatives of those 
Crinoids which lived in paleozoic and early mesozoic 
times are few in number, and for the most part live in deep 
water, or, as in the case of Rhizocrinus and its living allies, 
at great depths. They are like Limulus and Nebalia, rem- 
nants of an ancient fauna. There are but eight genera 
known—viz., Holopus, Rhizocrinus, Bathycrinus, Hyocri- 
nus, Pentacrinus, Comaster, Actinometra, and Antedon 
(Comatula). Of the first five genera the species are attached 
by a stalk to the sea-bottom, while the last three genera are 
in their young state stalked, but finally become detached. 
The body or calyx divides into arms bearing pinnule or sub- 
branches.. 
The Pentacrinus lives attached to rocks from twenty to 
thirty fathoms below low-water mark in the West Indies. 
The stem is about a foot long, the joints pentagonal, send- 
ing off at intervals whorls of unbranched cirri.“ No dis- 
tinct basal piece is known, but the calyx appears to begin 
with the first five radialia ’’ (Huxley). Pentacrinus ca- 
put-meduse Miller (Fig. 127) and P. Millert Oersted are 
West Indian species. P. Wyville-Thompsoni Jeffreys was 
dredged in deep water on the coast of Portugal. In the 
fossil P. subangularis the stalk was more than fifty feet long. 
Bathysrinus gracilis Wyville-Thompson is closely allied 
