OPHIOPHOLIS ACULEATA 185 



surface of the animal the disc is reduced to the interradii appear- 

 ing as pouches between the arms, which extend to the mouth and 

 enter into the formation of the oral ring or pentagon. On the 

 inner surface of the pentagon are five interradial jaw plates each 

 carrying a series of teeth. The free edge of the pentagon is fringed 

 with scales called oral papilla. Immediately behind this fringe 

 in each interradius are two narrow adoral plates followed by a 

 large oral plate. One of the five oral plates has a minute pore and 

 functions as madreporic plate. On each side of the arms, extend- 

 ing from the oral pentagon almost to the edge of the disc is a 

 narrow genital slit. 



The first thing noticeable in the structure of the arms is the 

 absence of the ambulacral groove. Instead, the oral surface 

 of the arm is flat and presents a single row of plates one plate 

 for each segment of the arm the so-called "central or under arm 

 plates. The aboral surface of the arm presents a similar row of 

 plates the dorsal or upper arm plates. These plates are sep- 

 arated from each other by a row of small scales found also be- 

 tween the dorsal and lateral plates. Each lateral plate has five or 

 six movable spines. The openings for the ambulacral tentacles 

 are situated between the lateral and ventral plates, one opening 

 on each side. The tentacles are protected by a tentacle scale. 

 The dorsal, ventral and two lateral plates are ossifi cations rep- 

 resenting the outer skeleton of the arm. The arm of all brittle 

 stars has also an inner skeleton consisting of a row of articulated 

 vertebra occupying the major part of the cavity of the arm. On 

 removal of the aboral wall of the central disc one can see four 

 vertebrae of each arm between the oral pentagon and the edge 

 of the disc. Between them are the cavities of the interradii 

 for the reception of the genital organs and of the stomach 

 pouches. 



The internal anatomy is very similar to that of the starfish 

 and the various systems are built on the same principle. But 

 there are certain differences in structure, which should be 

 emphasized. The digestive system is restricted to the central 



