STARFISHES 61 



found near low-tide mark on most shores. The animal has a 

 star-shaped body, consisting of a central part called the central 

 or body disk, and five symmetrically arranged arms or rays, broad 

 at their base, tapering slightly towards their bluntly pointed 

 tips, the whole animal presenting somewhat the appearance of 

 a conventional drawing of a star. Examining it more closely, we 

 shall see that the Starfish has an upper or dorsal, and an under 

 or ventral surface (also called the aboral and oral surfaces), the 

 upper surface being convex and of deeper colour, the under 

 surface flat and of lighter tint. 



Turning the Starfish over on its back, we see in the centre of 

 the oral surface a five-rayed aperture which communicates directly 

 with the mouth, and from which radiate five narrow grooves, 

 each traversing the centre of the oral surface of one of the rays 

 throughout its entire length. These are the ambulacral grooves, 1 

 and bordering each there are two or three rows of movable spines, 

 called the ambulacral spines. Filling each ambulacral groove 

 can be seen the numerous tube-feet, soft tubular bodies capable 

 of being greatly extended, and terminating in sucker-like ends. 

 It is by means of these tube-feet that the Starfish creeps along the 

 floor of the sea, and is also able to right itself if turned over on 

 to its back. A small bright red spot, the eye, can be seen at the 

 extremity of each of the ambulacral grooves, and immediately 

 over it is a median process called the tentacle, similar in 

 appearance to the tube-feet, but smaller, destitute of a terminal 

 sucker, and really functioning as an olfactory organ. These 

 tentacles are much more important organs than the eyes, for the 

 Starfish is guided to its food far more by their means than by 

 the sense of vision. 



The internal skeleton of each ray consists of two longitudinal 

 series of plate - like joints, called the ambulacral ossicles, 

 which lean against each other in the middle line above, and so 

 form the sides and roof of the ambulacral groove. The external 

 skeleton is a network of rod-like plates embedded in the tough 

 leathery skin, while from the soft interspaces between the network 

 project innumerable minute transparent membranous pouches, 

 which are respiratory in function. 



Scattered about over both the upper and under surfaces of 



1 Latin, ambulacrum, a place for walking. 



