CLASS I. ECniNODERMATA: ORDER 2. ECIIINIDA. 



625 



sea-urchin: echinus esculentus. 



SHELL OP SEA-lJKCniN WITIIOLF THE SPINES. 



little sucking feet, or ambulacra, are protruded, being arranged upon five rows of plates. The mode 

 in which the capacity of the shell is increased is exceedingly curious and interesting. It is entirely 

 covered by a skin of greater or less thickness, and it appears that, in spite of the close proximity of 

 the edges of the plates, there is yet room for the passage of a minute layer of skin through all 

 their interstices. It is in this that tlie deposition of calcareous matter takes place, so that, 

 instead of adding fresh matter to the shell only at the oral aperture, as in the Mollusca, the ani- 

 mal increases the size of its domicile in proportion to its growth, by continual additions to the 

 edge of every plate of which it is composed. New plates are also often added in the neighbor- 

 hood of tlie superior orifice. Next to this peculiarity in the form of the shell, the most striking 

 character of the Echinida consists in the nuinerous spines, frequently of large size, with which 

 the shell is covered. Tiiese are articulated to the numerous tubercles presented by the surface 

 of the shell, the base of the spines being hollowed for the reception of the convex surface of the 

 tubercle — in fact, forming a kind of ball-and-socket joint. In consequence of this mode of attach- 

 ment, the spines possess a considerable power of movement ; they serve to bury their owner in the 

 sand when circumstances require this concealment ; and some species appear by the same means, 

 to excavate hollows even in hard rocks. The spines with the ambulacra are used for locomotion, 

 so that these creatures climb rocks and "thus reach the coralines and shell-fish on which they feed. 



The most remarkable family of the Echinida are the Sea-Eggs or Sea- Urchins — Cidaridce: 

 they consist of several species, somewhat varying in form, some being nearly flat, some oval, some 

 heart-shaped, and some like the Echinus esculentus, resembling an orange in shape. The globu- 

 lar crest of this animal is made up of several hundred polygonal pieces of different sizes, of every 

 variety of outline, and so accurately fitted to each other that the lines uniting them are nearly 

 imperceptible. The complicated structure of these creatures is calculated to excite profound 

 astonishment. " In a moderate-sized urchin, that is, about a foot in circumference," says Forbes, 

 " I reckoned sixty -two rows of pores in the ten avenues. Now as there are three pairs of pores 

 in each row, their number multiplied by six, and again by ten, gives the great number of three 

 thousand seven hundred and twenty pores; but as each sucker occupies a pair of pores, the 

 number of suckers would be half that amount. There are above three hundred plates bearing 

 on its surface four thousand spines, each having a free socket movement !" 



The Urchins are common in the Atlantic, and many have been found and described on the 

 British coasts. According to Forbes there are the various kinds called the Piper, Cidaris 

 papillata : the Common Egg-Urciiin, Echinus sphoera: Fleming's Egg-Urciiin, E ^ Flemingii : 

 the Purple Egg-Urciiin, E.lividus: the Silky-spined Egg-Urchin, -fe^. ncglcctus : the Green- 

 Pea Urchin, ^cAmocyamMs j!)Ms^7^MS ; the Q KKY.-^ ViCViYS, Echinarachnius jplacenta: the Heart- 

 Urchin or Mermaid's IIead, Amphidotus cordatus, &c., the last being the commonest European 

 species, and often called Child's Head Urchin, Hairy Sea-Egg, &c. This is of a yellowish-white 

 color, and measures about one inch and a half in diameter. Several species of Echinus are eaten. 



Vol. II.— Y9 



