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.
The entire apparatus removed from the shell is represented in fig. 96, and consists of the following parts: - There are five long teeth (c c), each of which is enclosed in a triangular osseous piece (a a), that for the sake of brevity we will call the jaws. The five jaws are united to each other by various muscles (k h, i i), so as to form a pentagonal pyramid, having its apex in contact with the oral orifice of the shell, while its base is connected with several bony levers by means of numerous muscles provided for the movements of the whole. These parts we must now proceed to describe seriatim. The teeth (fig. 97, 1, a) resemble, at the part protruded from the mouth, long three-sided prisms, and at this point they are extremely hard and brittle: each tooth is fixed in a socket passing through the jaw (fig. 97, 2, e), from which it projects by its opposite extremity (fig. 97, 2, a'), that may be called the root of the tooth, where, instead of being of glassy hardness like the point (a) which issues from the mouth, it is flexible and soft, resembling fibres of asbestos, and is covered by a membrane apparently connected with its secretion. The jaws, which thus support and partially enclose these teeth, are five in number: when examined separately, each is found to resemble in figure a triangular pyramid, the external surface (fig. 97,2,e) being smooth, and presenting eminences provided for the attachment of muscles; while the other two sides (fig. 97, 1, b b) are flat, and marked with transverse grooves, so as to have the appearance of a fine file. When the five jaws are fixed together in their natural positions, they form a five-sided conical mass, aptly enough compared by Aristotle to a lantern, and frequently described by modern writers under the name of the "lantern of Aristotle." When thus fitted to each other, the two flat and striated sides of each jaw are in apposition with the corresponding surfaces of two others, so that there are ten grinding surfaces formed, between which the food must pass preparatory to its introduction into the digestive canal. This arrangement will be easily understood by referring to fig. 97,1, in which three of these jaws, each containing its incisor tooth, are represented in situ.
Fig. 96. Oral apparatus of Echinus: aaaaa, pyramidal pieces forming the "lantern of Aristotle"; b b, internal projections from shell; cccc c, teeth enclosed in their sockets; d d, interposed osseous pieces; e e, curved processes; ff, gg, hh,ii,kk, muscular fasciculi for the movements of the jaws.
Fig. 97. Dental system of Echinus. 1. Represents three of the pyramidal pieces forming the " lantern of Aristotle " in situ: a a, cutting extremities of the incisor teeth, which are of enamel-like hardness; a' a' a', fibrous roots of the same, resembling asbestos in their texture; b b, opposed flat surfaces of the jaws; d d, arched processes. 2. An isolated pyramid: e, its external surface. Other letters as in fig. 1.
(512). The five curious jaws described above are fixed together by a set of muscles (fig. 96, k k), consisting of short fibres passing between the external edges of the contiguous segments of the lantern, and evidently capable of powerfully approximating the grinding surfaces and rubbing them upon each other. The jaws, moreover, are provided with five other osseous pieces (d d), arranged in a radiating manner between the bases of the different segments, with which they are connected by ligaments, and likewise by the pentagonal muscle (i i) that runs from one to the other.
(513). The above-described parts complete the apparatus required for connecting the different portions of this remarkable mouth; but the movements of the whole are effected by a very complicated set of levers and muscles, which must next be noticed.
(514). The levers attached to the jaws are five long and slender processes (fig. 97, 1, dd), each arising from the central extremity of one of the radiating osseous pieces (c c), and arching outwards considerably beyond the base of the lantern, to terminate by a forked extremity. But there are likewise other processes projecting from the inner surface of the shell; these, two of which are seen in fig. 96, b b, are also five in number, and are placed around the orifice of the mouth: they are generally perforated in the centre, so as to resemble so many bony arches; and from them, as well as from the spaces which separate them, numerous muscles derive their origin. Of these muscles, ten (ff) arise from the spaces between the arches, two being inserted into the outer edge of the base of each jaw; so that the effect produced by their contraction, when they all act in concert, will be to approximate the whole mass of the mouth to the oral aperture of the shell, and of course cause the points of the incisor teeth to protrude externally; or, if they act separately, they can draw the base of the lantern in any direction, or cause the grinding surfaces of the jaws to work against each other.
(515). The antagonists to the muscles last mentioned are ten others (g g), arising from the extremities of the arches themselves, and running in a radiating manner towards the apex of the lantern, so that the point of each piece or jaw receives a muscle from two of those processes. These fasciculi, from the manner in which the arches project into the cavity of the shell, will draw inwards the entire mass; or, if they act separately upon the jaws whereunto they are individually fixed, they will produce movements precisely opposite to those caused by the contractions of the muscles derived from the spaces between the bony processes; or, if both sets should act in concert, they become the antagonists of the muscles (i i, k k) that connect the jaws to each other, and by causing the separation of the different pieces they necessarily enlarge not only the opening of the mouth, but all the passage leading to the oesophagus through the axis of the lantern.