CRINOIDEA. 119 



surfaces often having a very elaborate structure, so that the 

 entire stem possesses in the living state a greater or less 

 amount of flexibility. Each joint is perforated centrally by a 

 canal, which is, very inappropriately, termed the " alimentary 

 canal/' but which in truth has nothing to do with the digestive 

 system of the animal. At the summit of the stem is placed 

 the body, which is termed the " calyx/' and which is usually 

 more or less cup-shaped, pyriform, bursiform, or discoidal. 

 The calyx exhibits two surfaces, a dorsal and a ventral, of 

 which the dorsal is composed of calcareous plates articulated 

 by their margins, whilst the former is composed of a more or 

 less leathery integument strengthened by the deposition in it 

 of numerous small plates of carbonate of lime. The ventral 

 surface exhibits the aperture of the mouth, which may be sub- 

 central or may be very excentric, 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 ex- 

 centrically in one of the spaces between the arms, and which 

 is generally, if not universally, 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 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 

 (ambulacral) systems ; but it does not contain the reproduc- 

 tive 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 articulations, and which are termed the "arms" 

 (% 73)- 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 graceful 

 filaments is formed. The arms carry smaller lateral branches 

 or " pinnulas " on both sides ; and they are not hollow like the 

 arms of the Star-fishes, nor do they contain any prolongations 

 of the stomach. The upper surface of the arms and pinnuke 

 is covered with a soft membrane, and below this are placed 

 the reproductive organs. The generative organs are, there- 



