XORTH AMERICAX STARFISHES. 117 



Luidia clathrata 



Luidia clalhrata LtJTK. 1859. Vidensk. Meddel. 



Asterias clathrata Say, 1825. Journ. Acad. Nat. Scien. Pliila. 



PI XX. 



The genera Astrojiecten and Luidia are most closely allied, not only 

 by their possessing but two rows of pointed ambulacra! suckers (PL XX. 

 Fig. 2), but also by the structure of the limestone network of the two 

 surfaces of the spines and other appendages covering them. As in Astro- 

 pecten, the actinal limestone network is limited to a small triangular 

 area close to the mouth in the angle between two arms ; this area reminds 

 ns of the interbrachial space on the actinal side covered by small plates in 

 such genera as Solaster and Crossaster ; with the latter they are closely 

 connected. The rest of the actinal surface of the arms is covered by 

 the narrow elongated marginal plates which correspond in number to the 

 ambulacral and interambulacral plates (PI. XX. Fig. 4). 



The actinal marginal plates are, as in Asti'opecten, separated at the 

 surface by deep grooves edged by minute spines less numerous along 

 the main lines of the grooves than in the grooves of Astropecten, but 

 much more crowded at the openings near the interambulacral plates, form- 

 ing a regular sieve from plate to plate. The spines when removed leave 

 upon the face of the plates markings exactly similar to those found as 

 bands upon Echini, and known as fascioles. The spines carried upon 

 these minute granules are similar in every respect to the spines of the 

 fascioles of Echini. Their function is evidently identical, namely, that of 

 filtering and clearing the water before it reaches the water-tubes. Their 

 use is much more apparent than in the Spatangoids, where the bands 

 of fascioles are really of use only when lining the edges of the sunken 

 ambulacra of such genera as Hemiaster, while their extension from the 

 tip of one ambulacral rosette to the other seems to be a remnant of a 

 structure having at the present day in Echini but little if any use, while 

 in Spatangoids it still performs its function of accumulating minute 

 muddy particles floating in the water, which would to a certain extent 

 impede the access of clean water to their lobed ambulacral tentacles. 

 I have not observed these fascioles in other genera besides Astropecten 

 and Luidia. The presence, however, in some genera of minute spines 

 arranged in tufts on a solid basis projecting above the general surface 

 shows us a regular transition from the closed area formed by them on 



