PHYLUM ECHINODERMATA 



161 



The dorsal arch is supported by a number of irregular 

 ossicles. The V-shaped ventral part of the body-wall — 

 i.e., the walls of the ambulacral groove — is supported 

 by two rows of elongated ossicles, the ambulacral ossicles 

 (Fig. 88, Amb. os), which meet together at the apex or 

 summit of the groove like the rafters supporting the roof 

 of a house, but with a movable articulation allowing of 



Fig. 87. — Starfish. Vertical section through an arm. amp, ampullae ; ep. epider- 

 mis ; rad. amb, radial vessel of the ambulacral system; s points to the septum 

 dividing the blood-vessel into two parts; rad. tie, radial nerve of the epidermal 

 system; sp, spaces in mesoderm of body-wall: t.f, tube-feet. (From Leuckart, 

 after Hamann.) 



separation or approximation of the two rows so as to open 

 or close the groove. Between the ambulacral ossicles of 

 each row are a series of oval openings, the ambulacral pores, 

 one between each contiguous pair of ossicles. In the ventral 

 groove lie the tube-feet (t.f). Each tube-foot is found to 

 correspond to one of the ambulacral pores. When the tube- 

 foot is drawn upon, it is seen to be continuous with one of 



