SUMMARY OF IMPORTANT FACTS 



309 



2. Farther characters are the existence of a ccelom, the ambulacral 

 system, and the mesodermal spiny skeleton, which has given the name 

 to the phylum. 



3. The ambulacral system is locomotor and occurs nowhere else. 

 It consists of a sieve-like madreporite (not always present), which passes 

 water to the stone canal, and from this to the ring canal and the five 

 radial canals to fill the ampulla and ambulacra. Lateral branches supply 

 the tentacles and cause their extension. 



"%"' I- . aN^tfvxt* s ^^^MV M^-^-i - 



FIG. 310.- 



"ucumaria frondosa, sea cucumber (from Emerton). 



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

 canals of the ambulacral system; stone canal, madreporite, septal organ 

 and genital ducts are interradial. 



5. The Echinoderma are divided into five classes: (i) Asteroidea, 

 (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 cssca extend. The ambulacral groove 

 open. 



