224 PALEONTOLOGY. 



Their palaeontological history is imperfectly known ; about 

 twenty species are found in the oolitic and cretacous rocks, 

 referable to four genera; D'Orbigny has found in the chalk 

 two hundred and fifty species, forming thirty genera ; and in 

 the tertiary beds four hundred and sixty species, belonging 

 to fifty-five genera. Their greatest development has been 

 reserved for the modern period, for the same author has found 

 in our seas nine hundred species, belonging to sixty-eight 

 genera.* 



THE ECHINODEBMATA. This class forms the true type of 

 the radiata, and is composed of animals, fixed and free, with a 

 highly organised integument, for the most part armed with 

 moveable spines. In the rayed families, the organs of loco- 

 motion are disposed around a central axis. In the spherical 

 forms they are ranged in rows like the lines of longitude on 

 a terrestrial globe, and the mouth and the anus are situate 

 at the opposite poles. Each element of the body is in 

 general repeated five times. Thus, the sea-lily has five 

 primary arms ; the sea-star, five rays ; and the sea-urchin, 

 five pairs of perforated, and five pairs of imperforated plates 

 in its shell. The external surface of the skeleton supports a 

 series of moveable spines, and the perforated portion gives 

 passage to thousands of tubular feet for gliding over the bed 

 of the shallow shores they inhabit. The three higher orders 

 of this class, which have the holothuria, echinus, and asterias 

 as their types, are free ambulatory animals, whilst the genera 

 of the crinoid order are for the most part fixed by a cal- 

 careous stem like the polypifera. The higher forms' possess 

 visual organs. In the sea-star they are situate at the extre- 

 mities of the rays. In the echinus, the mouth is provided 

 with five complicated jaws armed with five long teeth, and 

 the apparatus is moved by many pairs of muscles ; the intes- 

 tinal tube makes a tortuous course around the shell, and is 

 maintained in situ by a delicate mesentery, and terminates 

 at the upper pole of the sphere. In the sea-star, the 

 stomach is a capacious sac, with glandular appendages which 

 branch into each ray ; and in the sea-lily it reposes in the 

 calyx surrounded by the arms. 



The echinodermata have a distinct system of vessels for the 



* Pictet, torn. iv. p. 216. 



