OPHIUROIDEA. 



195 



covered by a soft integument, but in the Cladophiurae and 

 some Streptophiuridae the skeletal plates of both the disc and 

 the arms are much reduced, and the integument is soft and 

 thick. 



The skeleton of the arms consists of a double row of internally- 

 placed ossicles which are generally fused with each other in 

 pairs and are comparable to.the ambulacral ossicles of Asteroids 



FIG. 140. Transverse section through the arm of an Ophiurid, diagrammatic (from Lang). 

 1 tube-foot (anibulacral tentacle) ; 2 its cavity ; 3 epineural circular canal ; 4 circular 

 ganglion, both at the base of the tentacle ; 5 under arm plate ; 6 radial epineural canal ; 

 7 radial nerve trunk of the superficial oral system (ambulacral nerve) ; 8 radial blood- 

 vessel ; 9 radial trunk of the deep oral nervous system (Lange's nerve) ; 10 radial peri- 

 haemal canal; 1 1 peripheral branch of the radial nerve ; 12 spine ; 13 lower intervertebral 

 muscle ; 14 lateral plate ; 15 ambulacral (vertebral) ossicle ; 16 upper intervertebral 

 muscle ; 17 brachial coelom ; 18 specially ciliated strip of peritoneum ; 19 upper arm 

 plate ; 20 radial water- vascular trunk ; 21 lateral portions of the brachial coelom which 

 are repeated in each brachial segment ; 22 branch of water-vascular trunk to tube -foot ; 

 23 ganglion at base of spine ; 24 motor nerve from deep oral nervous system. 



(Fig. 140, 15). The water-vascular trunk, perihaemal canal 

 and nerve cords lie on the lower side of these ossicles and a 

 prolongation of the perivisceral space on the dorsal (17). 



The outer or ambit al skeleton of the arm is segmented. Each 

 segment consists of four plates ; a median under arm-plate, a 

 median upper arm-plate, and two lateral arm-plates (Fig. 140). 

 These plates join one another and the corresponding plates of 



