402 THE HOMOLOGIES OF PEDICELLARLE. 
upper coating of the arms of a starfish, the envelope of a Holo- 
thurian, the shell of a sea-urchin, is made up of Fig. 9. 
n irregular network of limestone cells (Fig. 
88) ; with increasing size this network becomes 
closed at certain points and sends off upright 
shanks which little by little form very irregular 
fan-shaped spines (Figs. 89 and 90); in our | 
common sea-urchins these spines are immova- 
ble, forming at that stage part of the test 
itself. As the spines grow they become more 
pointed (Fig. 91) but are still immovable. In 
somewhat more advanced stages a slight con- 
striction is formed at the base of the spine 
(Fig. 92) and very soon after that, below the 
constriction a tubercle is formed upon which 
the spine is articulated and capable of a : 
certain amount of motion by means of the muscular sheat 
Fig. 92. connecting the base of the spine and the tubercle, 
which fit by a ball and socket joint (Fig. 93); piso. 
soon the spine appears longitudinally stri- 
ated, the limestone cells of which it was 
composed when smaller being obliterated by 
the successive circular layers of the older 
spine (Fig. 94). 
some sea-urchins (Arbacia) we find 
spines which never become articulated, are always fixed, 
Fig. 93. and remind us of the embryonic stage of 
the spines of our common sea-urchin. In 
| : , one of the Echini discovered by M. Pour- 
> San © talés the fixed spines cover the whole 
upper part of the test (Fig. 95), the movable spines being 
Fig.%. limited to a circumscribed area along the edge of 
the shell (Podocidaris). : 
If we trace the development of the spines of star- 
fishes, we find something similar; but as the pedi- 
cellariæ are clustered round the base of the longer 
spines, we are able to distinguish in the earliest 
stages what will become a spine, and what will 
eventually form pedicellariæ, a distinction which it is not possible 
to make in Echini where the pedicellariz and spines are irregularly 
Fig. 90. 
