350 ECHINODERMA. 



Summary of Important Facts. 



1. The ECHINODERMA share the radiate structure with the 

 Coelenterata, but differ from them (a) in the numerical basis of 

 the symmetry (five) ; (b) in that, as embryology shows, they have 

 descended from bilateral forms. 



2. Farther characters are the existence of a coelom, the 

 ambulacral system, and the mesodermal spiny skeleton, which has 

 given the name to the phylum. 



3. The ambulacral S3^stem is locomotor and occurs nowhere else. 

 It consists of a sieve-like plate, the madreporite (not always pres- 

 ent), which passes water to the stone canal, and from this to the 



FIG 339,Oligotrochus vitreus.* (After Danielssen and Koren.) 



ring canal and the radial canals to fill the ampullae and ambulacra. 

 Lateral branches supply the tentacles and cause their extension. 



4. Blood-vessels and nerve cords run in the same radii as the 

 radial canals of the ambulacral system; stone canal, madreporite, 

 ovoid gland, and genital ducts are interradial. 



5. The Echinoderma are divided into five classes: (1) Aster- 

 oidea, (2) Ophiuroidea, (3) Crinoidea, (4) Echinoidea, and (5) 

 Holothuroidea. 



6. The ASTEROIDEA have a disc and (usually) five arms into 

 which the gastric pouches and hepatic caeca extend. The ambu- 

 lacral groove open. 



7. The OPHIUROIDEA also have disc and arms, but the ambu- 

 lacral groove is closed and the hepatic caeca absent. 



8. The CRINOIDEA have a cup-shaped body bearing arms, 

 usually branching, with pinnulae, and a stalk, usually with cirri. 

 They are either temporarily or permanently attached. The 

 Crinoidea are subdivided into Eucrinoidea, Edrioasteroidea, 

 Cystidea, and Blastoidea. 



9. The ECHINOIDEA are usually spherical or oval, armored with 

 calcareous plates which extend as meridional bands from peristome 

 to periproct, five pairs of ambulacral and five of interambulacral. 



10. The ambulacral plates end at the periproct with a single 

 ocular plate; the interambulacral with a similar genital plate. 

 The madreporite is fused with one of the genital plates. 



