376 



ZOOLOGY 



SECT. 



their outer extremities. There are two surfaces one, the dboral 

 or abactinal, directed upwards in the natural position of the living 

 animal ; the other, the oral or actinal, directed downwards. The 

 aboral surface is convex, the oral flat ; the colour of the former is 

 much darker than that of the latter. 



In the centre of the oral surface (Fig. 301) is a five-rayed 

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

 manner are five narrow grooves, the ambulacral grooves, each extend- 

 ing along the middle of the oral 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 



ambulacral spines. At 

 the central ends of the 

 grooves the ambulacral 

 spines of contiguous 

 sides of adjacent grooves 

 form five groups, the 

 mouth papillce, 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 separ- 

 ating the oral from the 

 aboral surface. 



On the convex aboral 

 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 integument. 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 branchiae 

 or papulce (Fig. 305, tiesp. cos), which is capable of being entirely 

 retracted. 



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

 face is an aperture, the anus (Fig 310), 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 (mad). The presence of this structure interferes to some 



Fio,.--301. Starfish, ( Aster ias i-ubens). General view of the 

 oral or actinal surface, showing the tube-feet. (From 

 Leuckart and Nitsche's Diagrams.) 



