ECHINODERMATA. 55 



and we have the Sun-stars (Solaster) ; the angles 

 may be gradually filled up, the rays becoming 

 shorter and shorter, as in the Starlets (Goniaster, 

 &c.) ; and at length they may be quite lost as rays, 

 appearing only as the five angles of a disk, and 

 then we have the Bird's-foot (Palmipes). 



The starry form has now quite disappeared, but 

 changes still proceed in the same direction ; the 

 body, from being flexible, becomes invested with 

 strong plates clothed with moveable spines, and 

 the five-angled outline is more and more lost. 

 It is still discernible in the Cake-urchin (Echina- 

 rachnius) ; but in the true Urchins or Sea-eggs 

 (Echinus), the form is become almost globular, and 

 the chief traces of the quinary arrangement are in 

 the rows of minute holes, which radiate, in five 

 pairs of lines, from one pole to the other of the 

 globe. 



Again the texture of the skin changes ; it 

 becomes soft and fleshy ; the globose figure be- 

 comes columnar, at first short and thick, bu^ 

 gradually increasing in length until we find the 

 Class passing out of our view in forms which it is 

 hard to distinguish from true Worms. For a while, 

 as in the Sea-cucumbers (Pentactidce) , suckers run 

 down the body in five double rows ; but in the 

 Thyonidce this arrangement is lost ; and finally, in 

 the Synaptada, all traces of this fivefold radiism 

 disappear. 



I have just spoken of suckers; these are singular 

 organs, and highly characteristic of the ECHINO- 

 DERMATA. In the earlier families they are not 

 found ; and locomotion is performed chiefly by the 

 flexibility and prehensile character of the lithe and 

 slender rays. But in the Starfishes, Sea-urchins, 





