" The shell is made of numerous pieces, between the contiguous margin of which, the shell secreting tegument dips down, adding continually to the circumference of every piece, cretaceous parti cles, layer after layer, by which the superficial size of each pro gressively increases, whilst its form remains unchanged. The thousand pieces that compose the shell, thus simultaneously be come enlarged, and as they never change their figure or the relative proportions that they bear to all the rest, the entire shell expands without the slightest deviation from the given form, till it attains the limit of its growth.
" Examine well the spines. The base of every spine presents a smooth concavity or socket, which exactly fits one of the rounded tubercles already pointed out, upon the outer surface of the shell, and forms a perfect joint. No matter how minute the spines, be they as thin and delicate as the fine pile of velvet, such as exist in the Olypeaster and in other flattened genera, or large and club-shaped, as in Cidarites, the joint is equally com plete and movable in all required directions. In the last men tioned race, indeed, we see additional provisions made to give mire attachment, the centre of each tubercle being furnished with a little pit, from which arises a strong ligament that is im planted in the base of the appended spine to obviate all chance of dislocation.
" To understand the mode in which the spines are all produced, and fixed upon the body of the Eehinus, we must again refer to the soft tegument whereby the numerous pieces of the shell are all secreted. The skin, which is, in fact, the living substance of the creature's body, likewise constructs the spines, and if a section of one of these is prepared, taken from a recent speci men, each will be found, encrusted over with a layer of this soft fleshy membrane, as were the stems of corals, or Gorgonioe ; and in like manner, the investing film is able to secrete cretaceous particles, which are arranged, stratum investing stratum, with such art and regularity, that few more beauteous objects can be found, than one of these neglected spines."
" The suckers of the Echini, in all essential particulars, re semble exactly those of an Asterias, consisting of long projecti ble fleshy cylinders, which are protruded and rendered tense by water, or some other fluid injected into them, from an appara tus contained within the body. The degree to which the protu sible tube can be exerted will of course differ in every genus in proportion to the length of the locomotive spines, it being abso lutely necessary, that, however long the latter organs may be, the prehensile portion of the sucker shall be able to reach be yond them ; so that in the long spired genera, of which, in warm climates, some are found having spines several inches in length, the tubes, with their appended adhesive discs, resemble little cables thrown out to a distance, for the animals, in order to secure an anchorage, or ropes whereby to hoist themselves from ledge to ledge, of the steep rock, on which their food is found.
" These suckers that surround the mouth perform, besides, another duty, and may be compared to fishing lines, ready to seize upon such prey as comes within their reach and drag it to the mouth to be devoured."