120 



FASCIOLES OF STARFISHES. 



Fig. 13. 



cover these marginal plates, but more especially the minute spines 

 crowded upon the surfaces of the grooves g, g, g" , and g" form an effective 

 sieve, and in thus freeing the water from its impurities before it circu- 

 lates through the channels between the abactinal plates, act exactly- 

 like the fascioles of Echini. Only in Starfishes we can much more 

 readily see their great use in the economy of the animal, while their 

 action in the Echini is much less eflicient. In a profile view of a part of 

 the edge of the arm of Astropecten {Fig. 13), the openings, left for the 

 passage of the water, which are lined by these so-called fascioles are very 

 plainly defined. Ip and // are the lower and upper marginal plates, with 

 the deep grooves /' between the lower plates and the furrows /" be- 

 tween the upper plates, these furrows being completely arched over by 

 the minute spines acting as meshes of a sieve. At the angle of the 



junction between the shallower horizontal grooves 

 and the deeper vertical grooves a prominent open- 

 ing g is formed for the passage of the bulk of 

 the water, which is thus admitted to be sifted. 

 The lettering of Fig. 13 is the same as in Fig. 12. 

 In Luidia the only difference in the mechanism 

 of the fascioles is the greater number of openings through which the 

 water is admitted to circulate between the columnar plates covering the 

 abactinal surfiice and a part of the arms. In Fig. 14 we have a section 

 of the arm of a Luidia, corresponding to Fig. 

 12 of an Astropecten. The lettering is the same, 

 onl}^, there being a larger number of upper 

 marginal plates, the passages between them [g, 

 g\ g", g", g"") are more numerous. The lower 

 marginal plate alone is as prominent as in 



Astropecten. The articulation forms a continuous ring 

 round the arm, broken by the columnar plates sur- 

 mounted with their tufts of minute spines. These tufts 

 are so thick as to form a uniform shield almost solid 

 and unbroken on the abactinal surface of the arms. 

 Seen from below {Fig. 15), the deep groove / of 

 the lower marginal plate edged with minute spines, the fascioles, is well 

 shown. A view of the edge of the arm of Luidia {Fig. 16, correspond- 

 ing to Fig. 13 of Astropecten) shows the small rectangular areas into 



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