$ 71. THE ECHINODERMATA. 15 
Famity: Asrerorpaz, 
Genera : Luidia, Astropecten, Ctenodiscus, Archaster, Stellaster, Astrogo- 
nium, Oreaster, Pteraster, Asteriscus, Culcita, Ophidiaster, Chaetaster, 
Solaster, Echinaster, Asteracanthion. 
be Y 
ORDER III. ECHINOIDEA. 
The calcareous shell forms a spherical or discoid shield, composed of im- 
movable plates. The digestive canal is asymmetrical. 
Famity: Ecurnipaz. 
Genera: Echinus,' Cidaris. 
Famity: CLypeastrrpar. 
Genera: Laganum, Scutella, Encope, Rotula, Lobophora, Echinocyamus, 
Mellita, Echinanthus. 
Faminy : Sparancrpag. 
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 csophagus is 
surrounded by a calcareous ring, constituting the rudiment of an internal 
skeleton. The body is cylindrical. «The digestive canal, generally asym- 
metrical, : 
Famity: HoLoraurinas. 
Genera: Holothuria, Pentacta, Bohadschia, Cladolabes. 
Famity: Synaprinag. 
Genera: Synapta, Chirodota. 
ORDER V. SIPUNCULOIDEA.. 
The cutaneous envelope is coriaceous, and free from calcareous corpus- 
cles, There is no calcareous ring about the csophagus.. The body is 
cylindrical ; the digestive canal, usually asymmetrical. - 7 
Famity : Sreuncunipaz. 
Genera: Stpunculus, Phascolosoma, 
Famity: Ecurormaz, 
Genera: Thalasserva, Echiurus, 
