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.
At length, in the last division of the class, even the locomotive suckers are lost, and the only external resemblance left between the now worm-like body and the forms above enumerated, is met with in the radiating tentacula that surround the mouth. The apodous Echino-dermata ("Echinodermes sans pieds" of Cuvier) have indeed been expunged from the list of radiated animals by some modern writers; but in every point of their internal structure we shall find them offer too many points of similarity to permit of their expulsion from the class under consideration, although they evidently form the connecting link between the Radiata and the lowest families of the Annulose division of the animal kingdom. The genus Fistularia (fig. 86) strikingly exhibits an approximation to the outward form of the Annelida; and the anatomy of these creatures, which we shall afterwards consider, equally indicates the affinities that unite them.
Fig. 85. Echinus esculentus.
(444). We have already, when speaking of the general division of the Echinoder-mata, put the reader in possession of all that is satisfactorily known concerning the structure of the Crinoid* genera, - our knowledge of those singular animals being entirely derived from the exterior conformation of a few recent species, and from the mutilated skeletons of fossil Encrinites, which exist in such abundance in the limestone strata of our own country.
(445). Commencing, therefore, with the ASTERIDae1, we shall now enter at once upon the consideration of the anatomy of such species as have been most carefully examined, and merely notice incidentally the modifications which occur in the disposition of various organs in kindred genera.
(446). On examining a living Asterias, the outer covering of its body is found to be composed of a dense coriaceous substance, in which numerous calcareous pieces are apparently imbedded. The coriaceous integument is generally coloured externally with lively tints, and is evidently possessed of considerable irritability, as it readily shrinks under the knife, or upon the application of various stimuli. When cut into, it has a semicartilaginous hardness; and fibrous bands, almost resembling tendon in their aspect, may be seen to radiate from the centre of the body towards the extremities of the rays. There is no doubt that the movements of the rays are effected by the contractions of this fibrous membrane, and that, especially in the most polyp-like forms, as in Comatula and Gorgono-cejphalus, the irritable skin is the principal agent in effecting locomotion.
Fig. 86. Fistularia.
1 The name of this family and of its typical genus is derived from (447.) Besides the calcareous matter deposited in its interior, this outer covering of the Star-fish appears to furnish several secretions of different descriptions. The colouring matter upon its surface is no doubt one of these; as is a reddish fluid which exudes from the integument of A. rubens, and is of so caustic a quality as occasionally to produce great irritation of the skin in persons by whom individuals of this species are incautiously handled: moreover, in A. aurantiaca the whole animal is coated with a thick mucus, so dense and filamentous that it may be raised in thin films resembling a cobweb, and might easily be taken for a cuticular covering.
(448). The exterior of the body is generally rendered rough and uneven by various structures, either imbedded in the substance of the coriaceous skin or projecting from its external surface. We have already described the articulated pieces attached to the rays of Comatula and others, which seem to be the most perfectly developed forms of these cutaneous appendages. In the common Star-fish of our own coast, similar spinous processes, but composed of only one calcareous piece, are attached to the inferior margins of each ray, sometimes in several rows; and, being still moveable, they may be useful in seizing prey, or even as assisting in progression. Upon the dorsal aspect of the body are other calcareous projections, exhibiting a great variety of forms, so as to render the entire surface of the animal uneven and tuberculated.
(449). But the most remarkable appendages to the integument of the Asterias are minute bodies, which have been named by authors Pedicellariae, and have been looked upon by many naturalists as distinct animals, allied to Polyps in structure, and living parasitically upon Starfishes and other Echinodermata. Each of these curious processes consists of a short stem, fixed by one extremity to the skin of the Asterias, and terminating at the opposite end in two or three points resembling, in some respects, the prongs of a fork: the stem itself does not seem to be perforated by any canal; nevertheless the terminating points are found to be highly irritable, and quickly seize hold of any minute body placed between them. Some writers regard these bodies as organs of prehension, used under certain circumstances for fixing the animals that possess them; but, from their small size and general appearance, they seem but ill adapted to such an office.
(450). The skeleton or calcareous framework imbedded in the skin of the Asteridae is by no means the least remarkable part of their structure: this consists of several hundred pieces, variously disposed, and for the most part fitted together with great accuracy, being either firmly soldered to each other, as we have seen them to be in the formation of the calcareous box that constitutes the central portion of Ophiura, or united by ligaments, so as to allow of a considerable degree of motion to take place between them, as in the rays of Ophiura, Gorgonocejphalus, and other Asteroid forms.