182 



PHYLUM ECHINODERMATA. 



a calcareous sieve-like plate perforated by many pores the 

 secondary water-pores. Though in the adult most of the pores 

 of the madreporite lead directly into the stone-canal, some of 

 them open into the axial sinus. In the larva the primary 

 water-pore opens into the axial sinus. With later growth this 

 one pore becomes divided by folding into many, the epithelium 



of some of which becomes directly 

 continuous with the epithelium 

 of the stone -canal, but some 

 of the pores retain their direct 

 connexion with the axial sinus. 



The circumoral vessel bears 

 two kinds of appendages the 

 polian vesicles (p. 184) and 

 Tiedemann's bodies (p. 185). 

 The radial canals, which lie 

 between the heads of the ambu- 

 lacral ossicles in the angle formed 

 by their apposition (Fig. 129), 

 give off on each side as many 

 lateral branches as there are tube- 

 fcet. Each of these branches 

 goes to a tube-foot and is con- 

 nected by a canal which passes 

 between two adjacent ossicles, 

 with the internally placed am- 

 pulla. In some forms there are 

 two ampullae to each tube-foot 

 (many Astropectinidae). The 



radial canals end blindly at the end of the arm in the ocular 

 tentacle. 



The tube-feet are always pointed in the young, but in the 

 adult they often terminate in suctorial disc-like expansions. 

 Sometimes the distal feet of an arm are pointed and the 

 proximal suctorial. The pointed feet are tactile, while the 

 others are adhesive as well. The tube-feet have a well-developed 

 ectoneural nerve layer, and longitudinal muscles only, except 

 in the sucking disc where there are radial fibres, the contraction 

 of which brings about the adhesion of the disc. At the junction 

 of the lateral branches with the radial trunk there is a valve 



Flo. 133. Diagrammatic representation 

 of the water-vascular system of a 

 starfish (from Clans). Re circumoral 

 vessel ; Ap polian vesicle ; Stc stone- 

 canal ; M madreporite ; P tube-feet ; 

 Ap' ampullae of the same. 



