106 
LIFE, IN ITS LOWER FORMS. 
for the protrusion of the sucker-feet, and five pairs alter- 
nating with the former are inter-ambulacra!.* Both kinds 
are studded on their exterior surface with tubercles or 
strong warts, oach of which is crowned with a little globu- 
lar highly-polished bead. On every one of these beads 
played during life a spine with a hollow at its base, form- 
ing with its suppoi’ting spherule a ball-and-socket joint of 
perfect construction, the spine being kept in its place, and 
yet allowed great extent and freedom of motion, by means 
of muscles that bound its dilated pedestal to the surround- 
ing integuments. 
Professor Forbes informs us that in a moderate-sized 
Urchin there are sixty-two rows of pores in each of the 
ten avenues, and as there are three pairs of poi’es in each 
row, the total number of pores is 3720 ; but as each sucker 
occupies a pair of pores, the number of suckers is 1860. 
He says, also, that there are above three hundred plates 
of one kind, and nearly as many of another, all dovetailing 
together with the greatest nicety and regularity, bearing 
on their surfaces above 4000 spines, each spine perfect in 
itself, and of a complicated structure, and having a free 
movement in its socket. “ Truly,” he adds, “ the skill 
of the Great Architect of Nature is not less displayed in 
* The author of “ Tanks and tlieir Inhabitants,” having done ns the honour 
of quoting the above description, says : — “ Wo do not attempt to explain these 
terms, for they appear to be arbitrary, and certainly have nothing to do with 
walking, as the word would seom to imply.” The word ambulacrwni means not 
a walking organ , but a wolkvng -place, a walk or alley in a garden. The plates 
which carry the tubercles are comparable to the flower-beds of the garden, and 
the narrow spaces, pierced with pores, are the walks between. It is true, the 
walking organs, the suckers, arc protruded from these spaces, and from thence 
only, but we believe the name ambulacra was given long before it was known 
that these organs were connected with locomotion. 
