IX 



PHYLUM ECHINODERMATA 



347 



or abactinal, 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 five-fayed 

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

 manner are five narrow grooves, the ambulacral grooves, each 

 running 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 (i-mlndriat'id s)nncx. At the central ends of the grooves the 

 ambulacral spines of 

 contiguous sides of ad- 

 jacent grooves form 

 five groups, the mo-n.th * 

 papUloe, one at each 

 angle of the mouth. 

 External to the- am- 

 bulacral spines are 

 three rows of stout 

 spines, which are not 

 movable ; and a third 

 series runs along the 

 border separating the 

 ventral from the dorsal 

 surface. 



Qn the convex <h/-- 

 -y d 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 denned 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 branchiae 

 or papula 1 - (Fig. 280, jRes}}. cat), which is capable of being entirely 

 retracted. 



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

 face is an aperture, the amis (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, th< surface of which is marked by a number of 

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

 madreporite. Tne presence of this structure interferes to some 



Fit.. :>7<'>. Starfish. General view of the ventral surface, 

 showing the tube-feet. (From Leuckart and XitscheV 

 Diagrams.) 



