548 ECHINODERMATA — -ECHINOIDEA chap. 



dorsal tube-feet, and they become modified in order to fit them 

 for this function. The locomotor tube-feet are very small and 

 feeble compared with those of Echinus esculentus, but this is 

 comprehensible when it is recollected how little resistance the 

 yielding sand would offer to the pull of a powerful tube-foot like 

 that of the Eegular Urchins, for in order to move the creature 

 through the sand a multitude of feeble pulls distributed all over 

 its surface is necessary, and the locomotor tube-feet are exactly 

 fitted, both as to size and number, for this object. 



The principal points in which Clypeastroidea vary amongst 

 themselves are (1) the nature of the internal skeleton, (2) the 

 shape, and (3) the spines. 



Internal Skeleton. — In EcMnocyamus and its allies this con- 

 sists in each interradius of two simple partitions radiating out 

 towards the edge of the disc ; in La'ganum it consists of walls 

 parallel to the edge of the disc ; in Clypeaster, of isolated pillars. 



Shape. — In Echinocyamus the outline is oval and the test 

 comparatively high. In Clypeaster and its allies the outline is 

 pentagonal, and the test is swollen up into a blunt elevation in 

 the centre. In a large number of genera, however, the test is, as 

 in Echinarachnius, extremely thin and flat, and the outline may 

 be variously indented. A first indication of this process is seen 

 in Echinarachnius itself, but in Botula the edge is drawn out 

 into finger-like processes which are all interradial. In Mellita 

 these processes unite with one another distally so as to surround 

 spaces called " lunules," which appear as perforations of the test. 



The Classification of the Clypeastroidea adopted by Agassiz 

 is based chiefly on the degree of development of the internal 

 skeleton, and as this is of great physiological importance to the 

 animals we shall follow it here ; but since it was published the 

 remarkable discovery has been made of Pygastrides, a type pre- 

 viously known only from fossils. We must therefore recognise 

 two sub-orders : — 



Sub-Order I. Protoclypeastroidea. 



Anus on dorsal surface near apical pole. One species, 



Pygastrides relictus} with no "petals," from deep water in the 



Caribbean Sea. 



1 Loven, " On a recent Form of Eohinooonidae, " Bih. Svenska Akad. HavA. xiii, 

 Af. 4, No. 10, 1889. 



