170 



PHYLUM ECHINODERMATA. 



the adambulacral spines (Fig. 121, 8). The ambulacral ossicles are always 

 without any form of spinous appendage, and the tubes connecting the 



ampullae and tube-feet pass be- 

 tween them in such a way that 

 never more than one tube passes 

 between two successive ossicles. 

 When these apertures are ar- 

 ranged in a straight line on each 

 side, as usually happens, there 

 are two rows of tube-feet in each 

 ambulacrum (Fig. 119) (biseriate 

 arrangement). When on the 

 other hand the apertures are 

 alternately near to and remote 

 from the middle line of the ray 

 (Fig. 120), there appear to be 

 four rows of tube feet (quadrise- 

 riate arrangement, Asteriidae, 

 Heliasteridae). In certain fossil 

 starfishes (Palaeasteroidea) the 

 ambulacral ossicles of the two 

 sides are not opposite one an- 

 other, but alternate. In living 

 forms (Euasteroidea) they are 

 opposite. The ossicles of both 

 series increase in number with the 

 growth of the starfish in such a 

 way that the oldest, i.e. first 

 formed, ossicles are next the 

 mouth, while the youngest are at 

 the end of the arm. The young- 



MV 



IG. 119. Diagram showing the skeletal pieces 

 of the arm of a starfish with adambulacral 

 peristom and biserial tube-feet, when viewed 

 from the actinal surface (from Ludwig). .41, 

 A2, etc., first, second, etc., ambulacral ossicle ; 

 A'l, A' 2, A' 3, first, second, third adambula- 

 cral ossicle ; inner intermediate piece (oral 

 plate); MV inframarginal plates; T terminal 

 plate; VI, V2, VZ ventrolateral plates. 



est ambulacral ossicle lies on the 

 ventral side of the terminal plate. 



The superambulacral pieces, which may be mentioned here, are internally 

 placed and connect the ambulacral ossicles with the marginal plates 

 (Fig. 121, 5) ; they are found 

 in the Astropectinidae, many 

 species of Linckiidae, and in 

 some species of the Porcellanas- 

 teridae and Archasteridae. 



The first two ambulacra] ossi- 

 cles of each side are more or less 

 fused with one another (they 

 are represented separate in the 

 diagram) and form with the first 

 of the adambulacral series the 

 dental apparatus of the peris- 

 tom. When the adambulacral 

 pieces of this system are more 

 prominent than the ambulacral, 

 and form the buccal angles, the 

 peristom is called adambulacral (Fig. 119) ; when, on the other hand, the 

 .ambulacral pieces are the more prominent and the buccal angles are less 



FIG. 120. Diagram showing skeletal pieces of 

 the proximal part of the arm of a starfish with 

 ambulacral peristom and quadriserial tube- 

 feet (from Ludwig). Al, A2, etc., first, 

 second, etc., ambulacral ossicle ; A 1, A'2, 

 A'3, first, second, third adambulacral ossicle ; 

 o inner intermediate piece (oral plate). 



