PHYLUM ECHINODERMATA 



349 



ransverse Section of an Arm. If one of the arms be cut 

 across transversely (Fig. 277 and Fig. 280, B) and the cut surface 

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

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

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

 limbs of which are connected with the ventral ends of the dorsal 

 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 

 dorsal arch is supported by a number of irregular ossicles. It is 

 perforated by the numerous small dermal pores, through which the 

 dermal branchiaB pro- 

 ject. The Y-shaped ven- 

 'tral part of the body- 

 wall i.e. the walls of 

 the ambulacral groove 

 is supported by two 

 rows of elongate ossicles, 

 the ambulacral ossicles 

 (Fig. 280,^lm6.-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 al- 

 lowing of separation 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 terminal 

 ossicle. At the edges 



of the groove a row of ossicles supports the ambulacral spines and 

 prominent tubercles. Between the ambulacral ossicles of each row 

 are a series of oval openings, the ambulacral pores, one betwjeen 

 each contiguous pair of 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. /.) : 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. VWhen the tube-fool is 

 drawn upon, it is seen to be contiguous 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 ampullw 



FIG. 277. Starfish. Vertical section through an arm ; 

 amp. ampullae ; ep. epidermis, r</. <inti>. radial vessel 

 of the ambulacral system, rad. H. r. (erroneously so 

 lettered) points to the septum dividing the blood- 

 vessel into two pai'ts. rad. ne. radial nerve of the 

 epidermal system, sp. spaces in mesoderm of body 

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



