THE ECIIINIDEA. 495 



occur in any given ambulacrum, and those which lie within a 

 semita are always different from the others. 



In the Clypeastroida, the petaloid portions of the ambu- 

 lacra possess branchial pedicels, interspersed with delicate 

 locomotive pedicels, provided with a calcareous skeleton and 

 with a terminal sucker. The latter kind alone extend on to 

 the oral portions of the ambulacra. 



The circumoral nerve of Echinus surrounds the oesoph- 

 agus near the mouth. It has a pentagonal form, and is 

 inclosed by the alveoli, between which the ambulacral nerves 

 pass, over the peristome and through the arches of the au- 

 riculae, to the ambulacra. Each ambulacral nerve is accom- 

 panied by a neural canal, which, however, insheathes the 

 nerve, and does not merely lie on its inner side. 1 



The only known organs of sense in the Echinidea are the 

 pigmented " eye-spots," developed in connection with the 

 ends of the ambulacral nerves. 



The peritoneal space is filled by a corpusculated fluid, 

 which is kept constantly in motion by cilia distributed over 

 the parietes and the contained viscera. The aeration of this 

 fluid appears to be facilitated in all the Echinoida, except 

 Cidaris, by five pairs of special branchial plumes developed 

 from the peristome; while, in the Clypeastroida and Spa- 

 tangoida, which possess the modified pedicels commonly 

 termed ambulacral gills, there are no such organs. 



In the Echinidea, a circular pseud-hsemal vessel, whence 

 branches are given off to the genitalia, is said to surround 

 the anus. The alimentary canal is accompanied by two ves- 

 sels, one on the side of the mesentery (dorsal), the other on 

 the free side (ventral), which communicate with a lacunar 

 network in its walls ; and besides these, a fusiform body run- 

 ning parallel with the madreporic canal, and terminating 

 inferiorly in a circular vessel which lies close to the circular 

 ambulacral vessel, around the oesophagus, has been described 

 as a "heart." 2 



The genital organs are sacculated glands, which attain a 

 large size in the breeding season, and open externally bv the 



1 Teuscher. I. o. 



2 According to Hoffmann's latest investigations, there is neither anal nor 

 oesophageal circular vessel in Sjmtangus and Echinus. In the former, a distinct 

 anastomotic trunk connects the intestinal vessels with the circular ambulacral 

 vessel. In the latter, both intestinal vessels open directly into the circular 

 ambulacral vessel, and what has been described as a heart is really the madre- 

 poric canal. ( u Ueber das Blutgefass-System der Echiniden," " Niederlan- 

 disches Archiv," Bd. i.) 



