ECHINODERMATA. 3 



water finds its way to the circular water-vascular canal sur- 

 rounding the oasophagus. The tube-feet being restricted to the 

 ventral surface of the rays and disk, that surface is distinguished 

 by the term 'ambulacra!' from the dorsal and lateral surfaces, 

 which do not possess tube-feet, and are therefore called ant- 

 ambulacral. Though the radial symmetry of the disk and arms 

 is apparently very evident, yet the animal is really bilaterally 

 symmetrical, thus : the madreporic tubercle is inter-radial, and 

 as both mouth and anus are central, a vertical plane drawn 

 through tubercle, mouth and anus, will divide the animal into 

 two symmetrical halves, each half including two rays and one 

 half of the bisected ray. The two rays between which the 

 madreporic tubercle lies are distinguished as the 'bivium/ while 

 the remaining rays constitute the 'trivium.' 



5. Another specimen, to shew the structure of the in- 

 vesting coriaceous test, and the disposition of the ossicles com- 

 posing it. 



The whole of the disk, with the exception of that portion on 

 which the madreporic tubercle is situated, has been removed: 

 and, with a single exception, the cavity of each ray has been 

 laid open by a longitudinal incision on the ant-ambulacral surface. 

 Four rays have been divided transversely. Each ray consists of 

 a solid axis and of the calcified arch supported by it. The axis 

 itself is composed of a longitudinal series of ossicles, apposed 

 to each other after the mariner of vertebrae, and forming a solid 

 though flexible rod extending along the ventral surface of each ray 

 from the mouth to the tip. Each ossicle is double that is, it is 

 composed of two lateral halves, closely applied in the mesial line, 

 and retained in position partly by a tendinous investment and 

 partly also by the longitudinal muscular fibres, which, in the 

 fresh condition, pass from ossicle to ossicle. Each lateral ossicle 

 gives off from its outer side a transverse process, which is 

 slightly sinuous, or emarginate on opposite sides: so that by 

 the apposition of the concavities of contiguous processes two 

 alternating rows of foramina for the exit of the tube-feet are 

 formed on each side of the median ossicles. The external 

 extremity of each of the transverse processes abuts upon a 

 small ad-ambulacral ossicle, which in addition to contributing 

 to form the prominent margin of the ambulacral groove, and 



12 



