SECT. vi. ECHINODERMATA. 183 



low limestone, and attained their maximum in the Creta- 

 ceous strata. A species of Diadema, with annulated hol- 

 low spines, common in the Chalk, still exists. Numerous 

 species of the genus Clypeaster, remarkable for their 

 flattened form, and known as lake urchins, are peculiar 

 to the Tertiary strata and existing seas ; and, lastly, five 

 species of Spatangidse, heart-shaped urchins, which lived 

 in the Tertiary periods, still exist. In consequence of 

 the porous texture of the solid calcareous parts of the 

 Echinidse, their fossil remains are commonly impregnated 

 with pyrites or silex, without altering their organic 

 structure, so that they exhibit a fracture like that of 

 calcareous spar. 



Echinodermata Holothuro'idea. 



The Holothuridse, or Sea-Cucumbers, are of a higher 

 organization than the preceding Echinoderms. They 

 are soft, worm-shaped, five-sided animals, covered by a 

 flexible, leathery integument or skin, in which are im- 

 bedded a vast multitude of microscopic calcareous plates 

 of reticulated structure. The mouth, which is placed at 

 one end of the animal, is surrounded by ten bony plates 

 form ing a lantern, analogous to that of the Echinus; they 

 support branching, tubular, and retractile tentacles, which 

 encompass the mouth like a star. The tentacles are con- 

 nected with sacs at their bases, and are extended and 

 retracted by the injection of a watery liquid contained 

 in them. Innumerable tubular, suctorial feet, precisely 

 similar to those of the Echinus, are protruded and re- 

 tracted through corresponding pores in the skin of the 

 animal by a watery liquid, in sacs, at their bases. The 

 water is supplied by a system of canals connected with an 

 annular reservoir round the top of the gullet, which is sup- 

 plied with water by a bottle-shaped bag at the mouth. 



Besides transverse muscles, five pairs of muscles at- 

 tached to the lantern at the mouth, extend throughout 



