XVIII 



CLYPEASTROIDEA 



545 



a " petal " (Fig. 243, A, p), for the respiratory tube-feet are arranged 

 in two rows which diverge from their commencement at the 

 " calyx " and slightly converge again towards the outer margin 

 of the disc, and thus in a dried specimen the two rows of double 

 pores outline an area having some resemblance in shape to the 

 petal of a flower. Besides these double pores for the larger tube- 

 feet there are numerous small single pores for the smaller 

 tube-feet ; these are found in all the plates, ambulacral and inter- 

 ambulacral, of the dorsal surface, but in the neighbourhood of 

 the grooves only on the ventral side. 



ami). 



cmib. 



Fig. 243. A, aboral, B, oral view of Echinarachnius parma after spines have been 

 removed, amb (in A), Ambulacral plates, (in B), ambulacral furrows ; g.p, genital 

 pore ; inter, interambulacral plate ; p, petal ; t.t, pore for terminal tentacle, x ^. 



The sphaeridia are only present to the number of one in each 

 radius. Each sphaeridium is enclosed in a pit situated near the 

 edge of the peristome. 



A remarkable feature in the skeleton of Echinarachnius 

 which is characteristic in greater or lesser degree of all Clypeas- 

 troidea is the presence of vertical partitions of calcareous matter 

 traversing the coelom and stretching from the upper to the lower 

 surface of the test. These are found principally in the peripheral 

 region of the animal ; and there can be no doubt that they have 

 originated as cellular bands traversing the coelom, for the forma- 

 tion of similar structures can be followed step by step in the 

 Crinoidea. In the axis of these trabeculae, or folds of the 

 coelomic wall, jelly is secreted, and into this the lime-producing 



vol. I 2 N 



