44 ECHINODERMATA. 



Brackina of Van Beneden is supposed to be a larval condition 

 of Astemcanthion rubens (Pivefinger of the oyster-dredgers), our 

 common species. Bipinnaria and Brachiolaria are among the 

 successive larval stages of the species of this order. 



Aster acanthiidce. Scytaster. Asterinida. 



Asteracanthion= *Palseaster. Asterina. 



Uraster Oreaster. Astenscus=Pal- 



Heliaster.' Asteriida. ^P 68 - 



Asterias = Astro- n 

 Solastride. pec ten. 



Echinaster = Cribella. Ctenodiscus. 

 Chgetaster. Luidia. Brisinga. 



Solaster. *Paleocoma. 



Class III. ECHINOIDEA. 



Body rounded or discoidal, enclosed in a shell composed of 

 numerous closely connected calcareous plates, and studded with 

 tubercles on which are jointed movable spines. Sexes distinct. 



Certain of the plates are perforated for the emission of the 

 tube-feet. These are the "ambulacra!" plates, arranged alter- 

 nately in pairs ; between each pair are the " interambulacral " 

 plates, also in pairs ; usually there are five pairs of each. The 

 anus is at the top, surrounded by two series of small plates, the 

 inner known as the "genital," the outer as the "ocular" plates; 

 each of the latter is perforated and "supports the eye-spot." 

 The mouth is below, and furnished with a single series of " oral " 

 plates. The latter has usually a very complicated arrangement 

 of jaws or 'teeth [the splanchnic skeleton]. The intestine is con- 

 voluted. There is a blood-vascular system, but no distinct organs 

 of respiration. 



Sea- water is admitted into the peritoneal cavity and into the 

 water-vascular system through the perforated madreporiform 

 tubercle, which is borne on the largest of the five genital plates. 



Except in Cidaridse, there are found in the ambulacral areas 

 certain minute hyaline ciliated bodies called " sphaeridia ; " Loven 

 supposes them to be sense-organs. 



In the Pluteus-stage of the larva there is an internal keleton 

 of calcareous rods as in the Ophiuroidea. 



The^first two orders of the following table form the Euechinoidea 

 of Bronn ; the last order is known as " Tessellata," and is now 

 extinct. 



