IX 



PHYLUM ECHINODERMATA 



347 



-V. 



or nl ,<i t 'final, directed upwards in the natural position of the living- 

 animal ; the other, the ventral, or actinal, directed downwards. 

 The dorsal surface is convex, the ventral flat ; the colour of the 

 former is much darker than that of the latter. 



In the centre of the ventral surface (Fig. 276) is a live-rayed 

 aperture, the actinostome, and running out from this in a radiating 

 manner are five narrow grooves, the ambulacral grooves, each 

 limning along the middle of the ventral surface of one of the 

 arms to its extremity. Bordering each of the ambulacral grooves 

 there are either two or three rows of movable calcareous spines, 

 the amln/Iacra/ *j>inc$. At the central ends of the grooves the 

 ambulacral spines of 

 contiguous sides of ad- 

 jacent grooves form 

 five groups, the month 

 papilla:, one at each 

 angle of the mouth. 

 External to the am- 

 bulacral spines arc 

 three rows of stout 

 spines, which are not 

 movable ; and a third 

 series runs along the 

 border separating the 

 ventral from the dorsal 

 surface. 



On the convex dor- 

 *< 1 1 surface there are a 

 number of short stout 

 spines arranged in ir- 

 regular rows parallel 

 with the long axes 

 of the rays. These 



are supported on irregularly-shaped ossicles buried in the in- 

 tegument. In the soft interspaces between the ossicles are a 

 number of minute pores, the dermal pores, scarcely visible with- 

 out the aid of a lens. Through each of these pores projects 

 a very small, soft, filiform process, one of the dermal brancJiia: 

 or papula: (Fig. 280, Eesp. cce), which is capable of being entirely 

 retracted. 



Very nearly, though not quite, in the centre of the dorsal sur- 

 face, is an aperture, the anus (Fig. 285), wide enough to admit 

 of the passage of a moderately stout pin. On the same surface, 

 midway between the bases of two of the rays, is a flat, nearly 

 circular plate, the surface of which is marked by a number of 

 radiating narrow, straight, or slightly wavy grooves : this is the 

 madreporite. The presence of this structure interferes to some 



Vic.. 27ii. Starfish. General view of the ventral surface, 

 showing the tube-feet. (From Leuckart and Xitsrhu '.- 

 Diagrams.) 



