§ 71. THE ECHINODERMATA. 75 



Family : Asteroidae. 



Genera : Luidia, Astropecteyi, Ctenodiscus, Ar chaster, StcUaster, Astrogo- 

 nium, Oreaster, Pteraster, Asterisms, Culcita, Ojjhidiasier, Chaetaster, 

 Solaster, Echinaster, Asteracanthion. 



ORDER III. ECHINOIDEA. 



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

 iDovable plates. The digestive canal is asymmetrical. 



Family: Echinidae. 

 Genera : Echinus, Cidaris. 



Family : Clypeastridae. 



Genera: Laganum, Scutella, Encope, Rotula, Lohophora, Echinocyamus, 

 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 cesophawus 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 : Holothicria, Pentacta, Bohadschia, Cladolabes. 



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, u:^'iaHv nFv^^-.netrical. 



Family : Sipunculidae. 

 Genera : Sipunculus, Phascolosoma. 



Family : Ecuiuridae. • 



Genera : Thcdassema, Echiurus. 



