1/2 MANUAL OF ZOOLOGY. 



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 excentrically 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 sup- 

 ported 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 reproductive organs, 

 as is the case with the visceral cavity of the other Echino- 

 derms. 



From the margins of the calyx, where the dorsal and ventral 

 surfaces join one another, arise 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" 

 (fig. 63). 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 "pinnulse" 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 pinnulae is covered with a 

 soft membrane, and below this are placed 

 the reproductive organs. The generative 

 organs are, therefore, not placed within 

 the calyx, and it follows of necessity that 

 there is no generative opening or " ovarian 

 aperture " in the walls of the calyx. The 

 ventral surfaces of the arms and pinnulae 

 are furnished with grooves, which in the 

 living species are seen to be covered with 

 vibratile cilia. The brachial grooves coalesce till they con- 

 stitute five primary grooves, which are continued from the bases 



