118 DESCRIPTIOX OF THE HAED PARTS OF STARFISHES. 



the abactinal surfiice between the marginal plates in such genera as 

 Astropecten and Luidia, to a somewhat looser arrangement in Solaster 

 endeca and Cribrella. This arrangement is still further modified in 

 Crossaster papposa and Pycnopodia, and leads to such spines as are 

 found in Asterina and Palmipes, where the tufts consist of a smaller 

 number of minute spines more uniformly scattered over the surface, thus 

 forming an approach to the usual distribution of spines in Asteracan- 

 thion. Finally we pass to genera where the spines are long and few in 

 number, and do not, as in the genera of the Asteriadae proper, perform 

 the part of sieves. 



In place of the single row of large marginal plates along the abactinal 

 edge of the arms, we find in Luidia a series of much smaller plates, coi*- 

 responding in number, as on the actinal side, to the number of ambula- 

 cral plates. There are sometimes four or five rows of such plates, form- 

 ing regular longitudinal and transverse rows (PL XX. Fig. 3), followed 

 towards the median band of the arm by more irregulai'ly arranged plates. 

 These plates form the base of prominent pillars, somewhat constricted in 

 the centre, flaring at the extremity, surmounted at the tip by short spines 

 or merely granules articulating in a shallow socket. These spines are 

 so closely packed as to leave but very narrow passages between ad- 

 joining rows (see PI. XX. Fig. 1), generally mere slits edged by 

 minute spines, so that longitudinal and transverse passages run the 

 whole length of the arms for the passage of water, which must be all 

 carefully sifted before it enters either through the passages protected by 

 the fiiscioles or through those screened by the minute spines of the abac- 

 tinal surface. 



The plates of the abactinal limestone networks are completely sol- 

 dered (PI. XX. Fig. 6), leaving but few irregular rows of holes for the 

 passage of the water-tubes to the abactinal side, where they are com- 

 pletely sheltered under the floor formed by the minute spines of the 

 abactinal surface of the arms. 



In no other genera of Starfishes do we find so great a simplicity in 

 the structure of the plates of the actinal ring as in Astropecten and 

 Luidia. Usually the ambulacral and interambulacral plates of the arras 

 differ in no essential way except at the actinal ring formed in most Star- 

 fishes by such a modification of the last joint as to make it somewhat 

 difficult to trace the homology of all the parts. This last joint is extremely 



