THE EOHINIDEA. 485 



these outgrowths are supported by a calcareous skeleton, 

 which is also bilaterally symmetrical. Metschnikoff ^ has 

 made the interesting observation that in an Ophiurid (prob- 

 ably Ophiothrlx fragills) the whole system of perivisceral 

 and ambulacral cavities arises from two bodies, one situated 

 on each side of the gullet, which are solid, though it is possi- 

 ble that they may primitively have been hollow diverticula of 

 the archenteron. Two cellular masses become detached from 

 these bodies, apply themselves to the sides of the stomach, 

 and are converted into disks, from which the parietal and vis- 

 ceral walls of the peritoneal cavity take their origin. The 

 rest of the solid body on the left side of the gullet acquires a 

 vesicular character, opens by a dorsal pore, and grows round 

 the gullet, to give rise to the circular ambulacral vessel. The 

 other solid body disappears. The mouth of the Echinopa^- 

 dium becomes that of the Ophiurid. 



It cannot be doubted that these solid bodies take their 

 origin, in the same way as in other Echinop^dia, from the 

 hypoblast ; and thus the question arises. How far does the 

 mesoblast thus formed differ from that which arises by the 

 mere outgrowth of cells from the hypoblast, as in the Dog- 

 fish, and how far does this case tend to render it probable 

 that a scliizocoele is only a modification of an enterocoele ? 



The EcniNTDEA. — An ordinary Sea-urchin is comparable 

 to a Holothurid, with the body distended into a more or less 

 globular forui, and with a skeleton in the form of regular 

 plates arranged in meridional series ; those plates which cor- 

 respond with the ambulacral vessels being superficial to the 

 latter, and consequently perforated by the canals which pass 

 from the ambulacral vessels to the pedicels. 



In the Echinidea^ as, for instance, in the ordinary Echinus 

 or Sea-urehin, the perisoma round the mouth [j^eristome) 

 is usually strengthened for some distance by irregular oral 

 plates. In addition, ten rounded plates are placed in pairs 

 close to the lip ; these support as many pedicels, and are per- 

 forated by the canals of the latter. A much smaller space 

 around the anus {periproct) is similarly protected by anal 

 plates. The rest of the body is supported by a continuous 

 wall made up of distinct, more or less pentagonal plates, usu- 

 ally firmly united by their edges, which is called the corona. 

 Of these plates there are twenty principal longitudinal series, 



^ " Studien ijber die Entwickelunix der Echinodermen und Nemertineu." 

 (" Mem. Acad. St.-Petersbourg," 1869.) 



