This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(430). The next class of beings which presents itself for our consideration seems, upon a partial survey, to be completely insulated and distinct from all other forms of living creatures; so peculiar is the external appearance and even the internal organization of the families composing it. The casual observer who should, for the first time, examine a star-fish or a sea-urchin, two of the most familiar examples of the Echnodermata met with upon our shores, would, indeed, find it a difficult task to associate them, either to the simpler animals we have already described, or to more perfect forms of existence hereafter to be mentioned; they would seem to stand alone in the creation, without appearing to form any portion of that series of development which we have hitherto been able to trace so continuously.
(431). But this apparent want of conformity to the general laws of development vanishes on more attentive examination; so that we may not only follow the steps by which every family of this extensive class merges insensibly into another, but perceive that, at the two opposite points of the circle, the Echinodermata are intimately in relation with the Polyps on one hand, while on the other they as obviously approximate the Annulose animals, to which the most perfectly organized amongst them bear a striking resemblance.
(432). It would be impossible within our present limits to do more than lay before the reader the most important types of structure exhibited by the Echinodermata; it must nevertheless be understood that innumerable intermediate families connect the different genera; so that, however dissimilar the examples we have selected for the purpose of elucidating their general habits and economy may appear, the gradations leading from one to another are easily discoverable.
We have already found that many tribes of Polyps secrete calcareous matter in large quantities, constructing for themselves the solid skeletons or polyparies which generally seem to be placed external to their soft and irritable bodies, but occasionally, as in Pennatula, within the living substance. Let us for a moment suppose a polyp supported upon a prolonged stem, and that, instead of depositing the earthy particles externally, they should be lodged in the substance of the polyp itself, so as to fill the pedicle, the body, the tentacula around the mouth, and all the appendages belonging to the animal, with solid pieces, of definite form; such pieces, being connected together by the soft parts, and surrounded on all sides with irritable matter, would thus form a complete internal skeleton, giving strength and support to the entire animal, and at the same time allowing flexure in every direction. A polyp so constituted would obviously, when dried, present an appearance similar to what is depicted in the annexed engraving (fig. 80), representing an Encrinoid Echinoderm in its perfect condition. That animals thus allied to polyps in their outward form have, in former times, existed in great numbers upon the surface of our planet is abundantly testified by the immense quantities of their remains met with in various calcareous strata; but their occurrence in a living state is at present extremely rare: one minute species only has been detected in our own seas*; while specimens of larger growth, such as that represented in the engraving, derived from tropical climates, are so seldom met with, that it is fortunate one or two examples have been found, to reveal to us the real structure of a race of animals once so common, but now almost completely extinct. The body of the Encrinus (fig. 80, a) (or pelvis, as the central portion of the animal is termed by geological writers) is composed of numerous calcareous pieces, varying in shape and arrangement, so as to become important guides to the identification of fossil species; from this central part arise the large rays (b b), each furnished with a double row of articulated appendages, which, as well as the arms, are no doubt instruments for seizing prey and conveying it to the mouth, situated in the centre of the body, near the point a. This part of the animal, when found in a fossil state, from its resemblance to a flower, has received the common name of a "lily-stone".
Fig. 80. Encrinus.
* Thompson (J. W.), Memoir concerning the Pentacrinus europceus. Cork, 1827, 4to.
(434). The body above described, with the rays proceeding from it, is supported upon a long pedicle (e), divided into countless segments; and upon the sides of the stem, similarly-constructed filamentary branches are fixed (d d) at equal intervals. The skeleton of an Encrinite consists, therefore, of thousands of regularly-shaped masses of calcareous earth, kept together by the living and irritable flesh in which they are imbedded; and it is to the contractions of this living investment that the movements of the animal are due; but after the death of the creature, and the consequent destruction of its soft parts, the pieces of the earthy framework become separated and fall asunder, forming the fossil remains called "Trochi" and known in the northern districts of our own island, where they are very abundant, as "St. Cuthbert's beads".
(435). Of the internal structure of the Encrinites nothing is satisfactorily known. That they possessed a distinct mouth and anal aperture is evident from the structure of the plates of the body; but this is the extent of our information concerning them*.
In order to convert an Encrinus into an animal capable of locomotion and able to crawl about at the bottom of the sea, little further would be requisite than to separate the body and arms from the fixed pedicle upon which they are supported; we should then have a creature resembling in every particular the Star-fishes. The Comatula, for example (fig. 81), one of the lowest of the asteroid Echino-dermata, might be looked upon as an animal thus detached. The central part, or body, which contains the viscera, is made up of numerous calcareous plates, having in its centre a stelliform mouth; and near this is a tubular orifice, probably to be regarded as an anus. Around the margin of the central disk arise five stunted arms; but these immediately divide into a variable number of long radiating branches, composed, like those of the Encrinus, of innumerable articulated earthy pieces enveloped in a living and irritable integument. We find, moreover, issuing from the sides of every one of the prolonged rays, a double row of secondary filaments, each containing an internal jointed skeleton, and capable of independent motion. The complicated arms of the Comatula, therefore, are not, like those of the Polyp, merely adapted to seize prey, but, from their superior firmness, may be used as so many legs, enabling the animal to travel fron place to place.