ECHINODERMATA : CRINQIDEA. 



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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." Owing to 

 the animal being supported 

 on a stalk, it is evident that 

 the " ventral " surface is 

 turned upwards, and the 

 " dorsal " surface down- 

 wards. The column springs 

 from the centre of the dorsal 

 surface; and a stalked Cri- 

 noid may therefore be com- 

 pared to a Star-fish turned 

 upside down, with its lower 

 or ambulacral surface supe- 

 rior, and its dorsal surface 

 looking downwards. The 

 calyx contains the digestive 

 canal, and the central por- 

 tions of the nervous and 

 water- vascular (ambulacral) 

 system ; but it does not con- 

 tain the reproductive organs, 

 as is the case with the vis- 

 ceral cavity of the other 

 Echinoderms. 



From the margins of the 

 calyx, where the dorsal and 

 ventral surfaces join one an-? 

 other, arises a series of longer 

 or shorter flexible processes, 

 which are composed of a 

 great number of small cal- 

 careous articulations, and 

 which are termed the "arms" 

 (fig. 102). 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 pro- 

 duced perhaps again dividing, until a crown of delicate grace- 

 ful filaments is formed. The arms carry smaller lateral 



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Fig. 101. Crinoidea. Rhizocrinus Lofotettsis, 

 a living Crinoid (after Wyville Thomson), 

 four times the natural size, a Stem ; b Calyx ; 

 c c Arms. 



