262 



CRINOIDEA. 



careous plates. The ventral surface exhibits the aperture of 

 the mouth, which may be subcentral or may be very excen- 

 tric, and which in many extinct forms is wholly concealed 

 from view. The ventral surface also exhibits the aperture 

 of the anus, which is usually placed excentrically in one of 

 the spaces between the arms, and which is often carried at 

 the end of a longer or shorter tubular eminence or process, 

 which is called the " proboscis. " Sometimes, on the other 

 hand, the anus is central, and the mouth is excentric. wing- 

 to the animal being supported on a stalk, it is evident that 

 the " ventral " surface is turned upwards, and the " dorsal " 

 surface downwards. The column springs from the centre of 

 the dorsal surface ; and a stalked Crinoid may therefore be 

 compared to a Star-fish turned upside down, with its lower 

 or ambulacral surface superior, and its dorsal surface looking 

 downwards. The calyx contains the digestive canal, and 

 the central portions of the nervous and water-vascular (am- 

 bulacral) systems ; but it does not contain the reproductive 

 organs, as is the case with the visceral cavity of the other 

 Echinoderms. 



From the margins of the calyx, where 

 the dorsal and ventral surfaces join one 

 another, arises a series of longer or shorter 

 flexible processes, which are composed of 

 a great number of small calcareous articu- 

 lations, and which are termed the " arms " 

 (fig. 151). The arms are usually primarily 

 five in number, but they generally divide 

 almost immediately into two branches, each 

 of which may again subdivide ; the branches 

 thus produced perhaps again dividing, until 

 a crown of delicate graceful filaments is 

 formed. The arms carry smaller lateral 

 branches or " pinnulse " on both sides ; and 

 they are not hollow like the arms of the 

 showing the lateral pin- Star-fishes, nor do they contain any pro- 



nulae. 



The 



lon^ations of the stomach. 



upper 



surface of the arms and pinnulae is covered with a soft mem- 

 brane, and below this are placed the reproductive organs. 



