ECHINODERMATA. OPHIUROIDEA. 71 



Between the lateral plates and the ventral there are 

 apertures for the passage of the tube-feet. The greater 

 part of the space enclosed by the four rows of plates is 

 filled up by a longitudinal row of ossicles (d) ; each ossicle 

 consists of two halves fused together along the median 

 vertical line and in this part is immovable. But each 

 ossicle in the row articulates with the preceding and 

 succeeding one, so that the arm is flexible. On the 



FIG. 17. Section of the arm of an Ophiuroid (Ophioglypha). a, dorsal 

 plate; b, lateral plate; c, ventral plate; d, ambulacral ossicles fused 

 along the median vertical line ; e, ambulacral groove. 



ventral surface of the row there is a groove (e) in which 

 the radial vascular and water-vascular vessels, and the 

 nerve cord are placed ; this corresponds to the ambulacral 

 groove of the star-fish and the plates above to the ambula- 

 cral ossicles. 



Coming now to the disc, this is usually round or 

 pentagonal. On its ventral surface (fig. 18 A) the arms 

 are prolonged so as to reach the mouth. The last pair of 

 ambulacral ossicles, those next the mouth, instead of 

 being fused like the others, become free, and each one 

 unites with that of the adjoining arm. The parts of the 

 disc that come between the arms are known as the inter- 

 brachial areas ; these are covered mainly with scales and 

 granules, but in each of the areas there is a large plate, 

 the buccal plate (b), and other smaller plates, projecting 



