CBmOlDS. 101 



a relict of an earlier period of development. In the Ophi- 

 urans the oral canal opens directly into the body-cavity ; 

 in EcUnothrix directly connects with the outer world by 

 means 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 difEerent classes. 



Class I. — Crinoidea {Stone-lilies, Encrinites, etc.) 



Order 1. BracMata. — The living representatives of those 

 Crinoids which lived in palaeozoic 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 hearing pinnulm 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 ofE 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-medusm Miiller (Eig. 64) and P. MiXlleri 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. 

 Bathycrinus gracilis Wyville-Thompson is closely allied 



