350 EGIIINOVERMA. 



Summary of Important Facts. 



1. The ECHINODERMA share the radiate structure with the 

 Cceleuterata, but differ from tliem («) in the numerical basis of 

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

 descended from bilateral forms. 



2. Farther characters are the e-xistence of a coelom, the 

 ambulacral system, and the mesodermal spiny skeleton, which has 

 given the name to the phjdum. 



3. The ambulacral system 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. — 0(iouti-ocJi.u-^ i'/^vu.s.* (After Danielsjsen 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 Eohinoderma are divided into five classes: (1) Aster- 

 oidea, (3) Ophiuroidea, (3) C'riuoidea, (4) Echinoidea, and (-5) 

 Holothuroidea. 



G. The AsTEROinEA have a disc and (usually) five arms into 

 which the gastric jiouches and hepatic c»ca extend. The ambu- 

 lacral groove open. 



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

 lacral groove is closed and the hepatic ca?ca absent. 



8. The Crixoidea have a cnp-slniped body bearing arms. 

 i^snally liranching, with pinnnbe, an.d a stalk, usually with cirri. 

 They are either temporarily or permanently attached. The 

 C'riuoidea are subdivided into Encriuoidea, Edrioasteroidea, 

 C'ystidea, and Blastoidea. 



0. I'lie Ec'HiNOiDK.v are usually splu'vical (u- oval, armored with 

 calcareous ^dates which extend ,-is mcriilidual l)an(ls from peristome 

 to periproct, five pairs of anilmlacral and five of iiiterambnlacral. 



10. The ambulacral i)lat,i>s vw\ at tlie periproct with a single 

 ocular plate; tlu' interambulacral with a similar genital plate. 

 The madrctporite is fused witli oni^ of tlie genital plates. 



