TYPE ECHINODERMA. 567 



dius. The stone-canal, as lias been noted, opens into a spe- 

 cial portion of the ccelom, the ampulla of the stone-canal, 

 and this again communicates with the exterior by a tube 

 opening by a pore placed in the adults on the oral surface 

 of the body in one of the buccal shields ; primitively the 

 opening is situated on the aboral surface, only later migrat- 

 ing to its final position. In Astrophyton, in which the buccal 

 shields are wanting, the madreporiform tubercle occurs on 

 the oral surface of the disk in one of the interradii, and in 

 some species there may be five tubercles, one in each interra- 

 dius, a multiplication of the pores and stone-canals occurring 

 also in other genera, such as Amphiura and Opliiolepis. 



In consequence of the position of the pore or tubercle the position of 

 the stone-canal in the Ophiuroidea is very different from that which it 

 possesses in other groups. Whereas in these it passes aborally from the 

 water vascular ring, in the Brittle-stars it hangs down from the ring 

 towards the oral surface of the body. All those structui-es too, such as 

 the axial sinus and the ovoid gland, which are usually associated with the 

 canal, undergo a similar transformation of position, which is possible on 

 account of the distance from the oral surface at which the water vascular 

 ring is situated (see Fig. 259). 



The digestive tract is very simple. The mouth guarded 

 by the oral angle-pieces opens into a short oesophagus (Fig. 

 260, 0], which communicates with a capacious saclike stomach, 

 slightly pouched out in each radius, but not extending into 

 the cavities of the arms. There is no anus in any member 

 of the group. 



The epithelial nervous system of the Ophiuroidea is asso- 

 ciated with the general ectoderm in very young specimens, 

 but later sinks into the cavity of the bod} r by a process 

 which may be compared to an invagiuation. Consequently 

 a tube is formed lying within the body-wall on the oral sur- 

 face of the body, the radial nerves (Fig. 260, nr] being 

 situated in its aboral wall. This tube forms the epineurai 

 sinus, and the cavity it encloses seems to be in reality a por- 

 tion of the exterior, though it may be schizocoelic. The 

 oral ring of the nervous system is not enclosed in an epi- 

 neural canal, but remains in connection with the ectoderm 

 at the lower extremity of the oesophagus, being pushed thus 



