NORTH AMERICAN STARFISHES 115 



arm is occupied by the marginal plates ; these project beyond the mar- 

 ginal plate of the abactinal surface ; forming, when seen from above, what 

 appears like a second row of marginal plates (see Fifj. 3, PI. XIX.). The 

 abactinal limestone network extends over the disk and over the narrow 

 elongate space left on the upper side of the arms between the marginal 

 plates (PI. XIX. Figs. 1, 3, G). The marginal plates are firmly soldered 

 together, leaving no space between the floors where they are placed, 

 with the exception of a single large opening' for the passage of water- 

 tubes along the line of junction of two plates, across the arms; the whole 

 space in the arms between the plates being thus reduced to a narrow 

 flattened space, of which the larger part is occupied by the ambulacral 

 plates (PI. XIX. Fig. 5). 



The interbrachial arches are reduced to a thin partition at the angle 

 of the arms, where the abactinal marginal plates attain their greatest 

 height. The abactinal limestone network is, when seen from above, found 

 to be closely covered by short club-like spines, often with a broad base 

 and constriction in the middle below the head, attaining their greatest 

 diameter a short distance from the base of the arms, passing gradually 

 into mere granules towards the extremity of the arms and the centre of 

 the disk ; these spines are attached to the abactinal limestone network 

 (PI. XIX. Fig. 3) by a very shallow sucker, shaped like a saucer, with 

 edges slightly turned up. On the tip of these spines are arranged concen- 

 trically a number of minute spines more or less cylindrical, with rounded 

 ends, often completely filling the interval between adjoining spines, so 

 that they appear to form at first glance a smooth surfiice (PI. XIX. Fig. 1) 

 over the whole space lying between the marginal plates. The grooves 

 between the adjoining marginal plates are lined by similar, but even 

 more delicate spines, which appear to perform the same functions as the 

 delicate spines on the fascioles of Echini, namely, to sift the foreign matter 

 contained in the water admitted to the water-tubes. 



Seen from the actinal side, the abactinal floor consists of small circular 

 plates (PL XIX. Fig. 6) corresponding to the flat saucer-like plates of 

 the centre of the disk ; seen from the opposite side, these gradually pass 

 into the flattened plates closely soldered together, which extend into the 

 arms, leaving only, however, on each side of the solid central band, a 

 number of passages for the water-tubes (see PI. XIX. Fig. 6). The gen- 

 eral surface of the marginal plates of the abactinal side is covered by 



16 



