112 DESCRIPTION OF THE HARD PARTS OF SOME 



fishes, or Starfishes in which the abactinal system is stiffened by heavy 



interbrachial partitions extending from the oral ring to the angle of the 



arms, or Starfishes where the abactinal reticulation is extremely solid, as 



in Ophidiaster, are all capable of but slight movements. On the contrary. 



Starfishes in which, as in most of the Asteracanthidas, the 

 rig. 10. .... 



ip , reticulation is loose, the interbrachial partitions reduced often 



Av^^^^j^ ^' to a film, or to a mere arch of limestone plates, and in which 



^' the ambulacral plates are high, are much more flexible. 



-oo 



'y<4.,.>>. The extremes are found in such forms as Anthenea and 

 '^ Brisinga. In some genera the arms are rendered more stiff 



by long flat spurs extending on the inner side of the actinal surface from 

 the sides of the ambulacral plates towards the edge of 

 the arms (see a b, Fig. 1, and a b, Fig. 8'). These spurs 

 are also highly developed in Cribrella {Fig. 10), where, 

 as in the preceding figures, they conceal the interam- 

 bvdacral plates, which are small compared with the 

 ambulacral plates of Culcita, Anthenea, Acanthaster, and 

 the like. The interambulacral plates retain their pre- 

 ponderance even towards the extremity of the arms 

 quite near the tip, where the ambulacral and interam- 

 bulacral plates become separated from the abactinal system proper (see 

 Fig. 11, the tip of an arm of Culcita). 



Solaster endeca. 



Solaster cnthra Forbes, 1839. Mem. Wern. Soc. 

 Asterias endeca Lin. 



PI. XVII 



In Solaster endeca the arrangement and general structure of the am- 

 bulacral and interambulacral plates are identical with those of Crossaster ; 

 tlio i)lates are, however, more closely articulated, and tlie basal ambu- 

 lacral plates attain a still greater prominence even than in Cros.saster. 

 The mouth-papilla) arc also more powerful. The fundamental difference 

 between tliese genera, Crossaster and Solaster, lies in the structure of the 

 abactinal lloor (compare PI. XVII. Fig. 1, and PL XII. Fig. 4). The ac- 

 tnial floor between the arms is composed of small, somewhat elongated 

 plates, arranged in niore or less regularly diverging rows, quite similar 

 to those of Crossaster. The interbrachial partitions can hardly be in- 



