IV. SUBKINGDOM ECHINODER- 

 MATA. 



G-eneral Characteristics. The Eehinoderrns (spiny- 

 skinned) have all the parts symmetrically arranged around 

 a central axis, thus necessitating a peculiar system of 

 radiating canals and nervous ganglia. They are all ovipa- 

 rous. They respire by a water- vasculary system permeating 

 the entire body. There is also a blood vascular system, with 

 a central contractile heart. 



(BODY CYLINDRICAL ; FREE ; MOUTH FORWARD. Holothuroide*. 



BODY SPHERICAL OR FLAT; FREE; MOUTH UNDERNEATH. Echinoidea. 

 BODY STAR-SHAPED ; FREE } MOUTH UNDERNEATH. Asteroidea. 



BODY CUP-SHAPED ; FIXED ; MOUTH UPPERMOST. Criiioidea. 



CLASS I. HOLOTHUROIDEA. 



General Characteristics. The Holothuroids (whole- 

 mouthed) have a tough skin, sprinkled with calcareous 

 granules. The mouth is surrounded with feathery tentacles, 



Fig. lAh. 



Pentaetafronddsa, Holothuroid or Sea-cucumber. 



and the body is worm-shaped, though constructed upon the 

 radiate plan. By means of their powerful longitudinal and 

 transverse muscles, they can disgorge their own stomachs, 

 and even throw off their tentacles, and, after remaining inert 

 for a season, take on a new set of the rejected organs. 



