106 



LIFE, IN ITS LOWER FORMS. 



for the protrusion of the sucker-feet, and five pairs alter- 

 nating with the former are inter-ambulacral.* Both kinds 

 are studded on their exterior surface with tubercles or 

 strong warts, each 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 supporting 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 pores 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. w Truly," he adds, " the skill 

 of the Great Architect of Nature is not less displayed in 



* The author of " Tanks and their Inhabitants," having done us the honour 

 of quoting the above description, says : — " We 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 seem to imply." The word ambulacrum means not 

 a walking organ, but a ico.lking-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, are protruded from these spaces, and from thence 

 ouly, but we believe the name ambulacra was given long before it was known 

 that these organs were connected with locomotion. 



