The British Star-Fishes. 251 



THE BEITISH STAR-FISHES. 



BY GEORGE S. BEADY, M.E.C.S. 

 (With a Tinted Plate.) 



It is not at all the object of the present paper to give any- 

 general account of the natural history of the Star-fishes, but 

 simply to notice a few points in their structure and economy 

 which may be interesting to the general reader, and especially 

 to the microscopist. And to be easily intelligible we must first 

 briefly describe the external structure and configuration of the 

 creatures to which we refer. 



The sub -kingdom Radiata of the older naturalists is now, in 

 accordance with the results of recent investigation, broken up 

 and reconstituted. The fundamental idea of a radiated struc- 

 ture was, however, so far as mere external appearance went, 

 correct enough, and no better type of such an organization 

 could be desired than that supplied by the common Star-fishes. 

 Each Star-fish consists of a central disk, from which are given 

 off symmetrically, like the spokes of a wheel, the arms, or to 

 speak technically, the rays. These rays are mostly five in 

 number, but in some species seven, twelve, or thirteen may be 

 observed. The common Sun-star (Solaster papposa), one of 

 the largest and handsomest, as well as one of the most common 

 species, has usually thirteen rays ; while the still commoner 

 Urasters, found everywhere between tide-marks, are examples 

 of the five-rayed form. The number five is, so to speak, the 

 key-note of the class Echinodermata, and may be found in all 

 parts of their external organization, continually recurring either 

 singly or in multiples. 



The under surface of a Star-fish exhibits a central orifice, 

 which acts both as mouth and anus ; and diverging from it on 

 the under surface of each ray is a longitudinal space or 

 s ' avenue,' - ' furnished on each side (except in the Ophiuradce, 

 Sand-stars and Brittle-stars, where moveable spines take the 

 place of suckers, and in which the " avenues" are absent) with 

 innumerable retractile suckers or feet, which serve as organs 

 of locomotion. The Star-fishes are divided according to their 

 mode of locomotion into Spinigrades, moving by means of 

 spines — Cirrhigrades, by suckers — and Pinnigrade, by fins or 

 pinnas. Of the last-named division we have only one British 

 genus, Oomaiula, concerning which some remarks will be found 

 toward the end of this paper. At the very extremity of each 

 ray is an organ like an eye, having spinous appendages, which 

 are termed the eyelids. It is doubtful, however, whether these 

 parts have really any visual endowment ; no proof of their pos- 

 sessing the faculty of sight has ever been advanced, and from 



