PHYLUM ECHINODERMATA 



371 



examined, the aboral part of the thick, hard wall of the arm will 

 present the appearance of an arch (with its convexity upwards), 

 and the oral part the form of an inverted V, the ends of the 

 limbs of which are connected with the oral ends of the aboral 

 arch by a very short, flat, horizontal portion. Enclosed by these 

 parts is a space, a part of the ccelome or body-cavity, and below, 

 between the two limbs of the V, is the ambulacral groove. The 

 aboral arch is supported by a number of irregular ossicles and is 

 perforated by the numerous small dermal pores, through which the 

 dermal branchiae project. 

 The V-shaped oral part 

 of the body- wall i.e. the 

 wails of the ambulacral 

 groove is supported by 

 two rows of elongated 

 ossicles, the ambidacral 

 ossicles (Fig. 310, 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 articu-' 

 lation allowing of separa- 

 tion or approximation of 

 the two rows so as to 

 open or close the groove. 

 At the end of the ray 

 the ambulacral ossicles 

 end in a median ter- 

 minal ossicle. At the 

 edges of the groove a row of ossicles support the ambulacra 

 spines and prominent tubercles. Between the ambulacral ossicles 

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

 between each pair of contiguous ossicles, and so arranged that they 

 form two rows on each side, one row higher than the other, the 

 pores of the higher row alternating with those of the lower. In 

 the ventral groove lie the contracted tube-feet (T. F.) : each tube- 

 foot is found to correspond to one of the ambulacral pores, so 

 that the former, like the latter, are arranged in a double alter- 

 nating row on each side of the groove. When the tube-foot is 

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

 little bladder-like bodies, which lie on the other side of the ambu- 

 lacral ossicles, i.e. in the cavity of the arm. These the ampullce 

 (Figs. 308 and 310, amp.) are arranged like the tube-feet, in a 

 double row on each side, a higher row and a lower, there being one 

 opposite each ambulacral pore. When one of them is squeezed, 



Fio. 308. Starfish. Vertical section through an arm. 

 amp. ampullae ; ep. epidermis ; rad. amb. radial vessel 

 of the ambulacral system ; rad. bl. v. points to the 

 septum dividing the perihspmal 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.) 



