VII 



PHYLUM ECHINODERMATA 



161 



arch is supported by a number of irregular ossicles. The 

 V-shaped ventral part of the body-wall i.e., the wall of the 

 ambulacral groove is supported by two rows of elongated 

 ossicles, the ambulacral ossicles (Fig. 82, Amb. os.), which meet 

 together at the apex or summit of the groove after the fashion 

 of the rafters supporting the roof of a house, but with a 

 movable articulation allowing of separation or approximation 



FIG. 81. Starfish. Vertical section through an arm. amp. ampullae ; ep. epi- 

 dermis ; rad. amb. radial vessel of the ambulacral system ; rad. bl. v. (errone- 

 ously so lettered) points to the septum dividing the blood-vessel into two parts ; 

 rad. ne. radial nerve of the epidermal system ; sp. spaces in mesoderm of body- 

 wall ; t.f. tube-feet. (From Leuckart, after Hamann.) 



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 make its exit from one of the 

 ambulacral pores. When the tube-foot is drawn upon, it is 

 Man. Zool. M 



