1 86 THE SCIENCE OF LIFE. 



gations of the body, and are not provided with suckers, 

 as the Ophiura, or Brittle-star, (Fig. 86,) and Asterophy- 



ton, or Basket-fish. The last 

 group are inferior in struct- 

 ure, and resemble inverted 

 stemless Crinoids. The di- 

 gestive sac is confined to the 

 disk, and the madreporic 

 plate is underneath. 



The order ECHINOIDEA, 

 FIG. se.-Ophiura. O r Sea-urchins, contains those 



Echinoderms whose skin secretes calcareous plates, form- 

 ing a hollow shell, covered with spines, and varying in 

 shape from a sphere to a disk. The shell of an Echinus 

 is made up of twenty rows, or zones, of plates, of which 

 five pairs are ambulacral, pierced with minute pores for 

 the protrusion of ambulacra, or sucker-feet, and five pairs 

 alternating with the former are inter-ambulacral. The 

 shell is developed from a membrane which lines the in- 

 terior of the plates, and passes between the joints, so 

 that additions can be made to their edges, by which 

 means the shell grows and preserves the same relative 

 proportions. The upper end of the shell, in addition to 

 five small circularly disposed plates, carries five large 

 genital plates. Each of these has a duct for the passage 

 of ova or spermatazoa, and an ocellus, or eye-spot. One 

 of these plates is the madreporic tubercle, with minute 

 apertures communicating with the madreporic canal. 

 Locomotion is effected by the hollow muscular feet, 

 each of which communicates with a water sac ; they 

 also communicate with each other, so that as each 



