43 2 ECHINODERMATA — ASTEROIDEA chap. 



semi-transparent, permitting the skeleton to be easily detected ; 

 this consists of a mesh-work of rods or plates, leaving between 

 them intervals of soft skin. In a living Starfish it can be seen 

 that many of these soft places are raised up into finger-like 

 outgrowths, which are termed " papulae " or " dermal gills," 

 through the thin walls of which an active interchange of gases 

 with the surrounding water takes place, and the animal obtains 

 in this way the oxygen necessary for its respiration. 



Very few and feeble muscle-fibres exist in the body-wall, and 

 the movements of the arms, as a whole, are very slow and limited 

 in range. There is a membranous lip surrounding the mouth, 

 from which five broad grooves run outwards, one on the under- 

 side of each arm. These are termed the " ambulacral grooves." 

 Each groove is A-shaped, and its sides are stiffened by a series of 

 rod-like ossicles called the " ambulacral ossicles." 



The animal progresses by the aid of a large number of trans- 

 lucent tentacles, termed " tube-feet " or "podia," which are attaclied 

 to the walls of the ambulacral grooves. 



Anatomy of a Starfish. — As an introduction to the study of 

 the anatomy not only of Starfish but of Echinodermata as a 

 whole, we select Astcrias ruhens, the common Starfish of the 

 British coasts, which in many places may be found on the beach 

 near low-water mark. 



External Features. — In this species (Fig. 185) the skeleton 

 is a net- work of rod-like plates, leaving wide meshes between 

 them, through which protrude a perfect forest of transparent 

 papulae. From the points of junction of the rods arise short 

 blunt spines surrounded by thick cushions of skin. The surfaces 

 of these cushions are covered with a multitude of whitish specks, 

 which, on closer inspection, are seen to have the form of minute 

 pincers, each consisting of two movable blades crossing eacli other 

 below and articulated to a basal piece. Tliese peculiar organs are 

 termed " pedicellariae " (Fig. 186), and their function is to keep 

 the animal clean by seizing hold of any minute organisms which 

 would attempt to settle on the soft and delicate skin. When 

 irritated the blades open and then snap together violently, and 

 remain closed for a long time.^ These actions are brought about 

 by appropriate muscles attaching the blades to the basal piece. 



^ Uexkiill, "Die Physiologie der Pedicellarien," Zeitschr. f. Biol, xxxvii. 1899, 

 p. 356. 



