FASCICLES OF STARFISHES. 110 



simple in Astropecten, being but slightly modified and difiering from the 

 others mainly in length. Thus the homology I have attempted to trace 

 between the jaws can there be seen in its simplest form (PI. XX. Fifjs. 

 4, 5, 8). The plates of the extremity of the arms are soldered together 

 when seen from above (PI. XX. Fifjs. 9-11), forming a prominent knob 

 with a deep groove on the actinal side for the passage of the ambulacral 

 tentacles. 



The spines of the actinal side increase slightly in length towardc the 

 outer edge of the arms, where there is found a prominent row of larger 

 flattened spines fringing the edge of the arms. 



The actinal face of the jaw-plates is prominent and thickly studded 

 with irregularly arranged minute spines, forming a marked feature at 

 the actinal angle of the arms, between adjoining ambulacral rows (PI. XX. 

 Fig. 7). 



The madreporic body is often irregular in outline (PI. XX. Fig. 12), 

 and is frequently completely hidden by the surrounding spines of the 

 abactinal surface. 



FASCIOLES OF STARFISHES. 



The description of the accompanying figures of Luidia and of Astro- 

 pecten will explain the disposition of the minute spines of those genera 

 which I have homologized with the fascioles of Echini. Fig. 12 repre- 

 sents a transverse section of an arm of Astropecten, 

 a being the ambulacral. ai the interambulacral, plate, 

 with its spines p. Ip is the plate on the edge of Kj^-^y^ 

 the lower side of the arm, and Ip the correspond- *^ 

 ing plate of the upper edge of the arm, tp being 

 the small columnar plates surmounted by tufts of 

 minute spines forming the close covering of the central part of the 

 abactinal side of the arm. The surfaces s of the upper and lower plates 

 on the edge of the arm are the articulating suriiices which rise somewhat 

 above the surrounding edge of the plate, leaving a flat space //' on the 

 lower arm-plate, g" on the upper arm-plate, and from g to g between these 

 two plates, through which water from outside can circulate as in a groove 

 all round the articulation, and thus find its way between the columnar 

 plates of the abactinal surfiice of the arm. The small papilla) which 



