^ 71. THE ECHINODERMATA. " • 75 



Family : Asteroidae. 



Genera : Luidia, Astropecten, Ctenodiscus, Archaster, Stellaster, Astrogo- 

 nimn, Oreaster, Fteraster, Asteriscus, Culcita, Ophidiaster, Chaetaster, 

 Solaster, Echhiaster, Asteracaiithion. 



ORDER III. ECHINOWEA. 



The calcareous shell forms a spherical or discoid shield, composed of im- 

 movable plates. The digestive canal is asymmetrical. 



Family : Echinidae, 

 Genera : Echinus, Cidaris. 



Family : Clypeastridae. 



Genera : Laganum, Scutella, Encope, Rotula, Lobophora, EcJiinocyamus^ 



Mellita, Echinanthus. 



Family : Spatangidae. 

 Genus : Spatangus. 



ORDER IV. HOLOTHURIOIDEA. 



In place of a calcareous shell, the cutaneous envelope contains a greater 

 or less number of calcareous reticulated corpuscles. The oesophagus is 

 surrounded by a calcareous ring, constituting the rudiment of an internal 

 skeleton. The body is cylindrical. The digestive canal, generally asym- 

 metrical. 



Family : Holothurinae. 



Genera : Holothuria, Pentacta, Bohadsckia, Cladolahes. 



Family: Synaptinae. 

 Genera : Synapta, Chirodota. 



ORDER V. SIPUNCULOIDEA. 



The cutaneous envelope is coriaceous, and free from calcareous corpus- 

 cles. There is no calcareous ring about the oesophagus. The body is 

 cylindrical ; the digestive canal, usually asymmetrical. 



Family : Sipunculidae. 

 Genera : Sipunculus, Phascolosoma. 



Family: Echiukidae. 

 Genera : Thalassetyi, Eckiurus. 



