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.
(716). The Annelidans examined in the preceding chapter, with the singular exception of the Earthworm, are only adapted to an aquatic life: the soft integument which forms their external skeleton, and the setiform and tentacular organs appended to the numerous segments of their elongated bodies are far too feeble to support them in a less dense and buoyant element; so that when removed from their native waters they are utterly helpless and impotent. Supposing, however, that, as a mere matter of speculation, it was inquired by what means animals of similar form could be rendered capable of assuming a terrestrial existence, so as to seek and obtain prey upon the surface of the earth, and thus represent upon land the Annelidans of the ocean, a little reflection would at once indicate the grosser changes required for the attainment of such an object. To convert the water-breathing organs of the aquatic worms into an apparatus adapted to aerial respiration would be the first requisite. The second would be to give greater density and firmness to the tegumentary skeleton, to allow of more powerful and accurately-applied muscular force by diminishing the number of segments composing the annulose covering, and also, by converting the lateral oars into jointed levers of support sufficiently strong to sustain the weight of the whole body, to provide instruments of locomotion fitted for progression upon the ground.
Yet all these changes would be inefficient without corresponding modifications in the character of the nervous system: the lengthened chain of small ganglia found in the aquatic worms would be quite inadequate to wield muscles of strength adapted to such altered circumstances; the small encephalic brain would be incompetent to correspond with more exalted senses; so that, as a necessary consequence of superior organization, the nervous centres must be all increased in their proportionate development to adapt them to higher functions.
(717). The changes which our supposition infers would be requisite for the conversion of an aquatic Annelidan into a land animal are precisely those which we encounter when we turn our attention from the creatures described in the last chapter to the Myriapoda, upon the consideration of which we are now entering: they form the transition from the red-blooded worms to the class of Insects, and are intermediate between the two in every point of their structure.
Ten thousand, i. e. many.
(718). The body of a Myriapod consists of a consecutive series of segments of equal dimensions, but, unlike those of the generality of the Annelida, composed of a dense semicalcareous or else of a firm coriaceous substance; and to every segment is appended one or two pairs of articulated legs, generally terminated by simple points.
(719). The anterior segment or head, besides the organs belonging to the mouth, contains the instruments of sensation, consisting of simple or compound eyes, and of two long and articulated organs called antenna, generally regarded as appropriated to the sense of touch, but which probably are connected with other perceptions less intelligible to us.
(720). The air required for respiration is taken into the body through a series of minute pores or spiracles placed on each side along the entire length of the animal, and is distributed by innumerable ramifying tubes or tracheae to all parts of the system.
(721). The number of segments, and consequently of feet, increases progressively with age, - a circumstance which remarkably distinguishes the Myriapoda from the entire class of Insects, properly so called.
(722). The Myriapoda may be divided into two families, originally indicated by Linnaeus: the Julidce, or millepedes, and the Scolopen-dridce, or centipedes, each of which will require our notice.
The lowest division, which derives its name from the Julus, or common millepede, is most nearly allied to the Annelidans, both in external form and also in the general arrangement of its different organs; this therefore we shall first examine, and select the Julus ter-restris, one of the species most frequently met with, as an example of the rest. These animals (fig. 139, a) are generally found concealed under stones, or beneath the bark of decayed timber, where they find subsistence by devouring decomposing animal and vegetable substances. The body is long and cylindrical, composed of between forty and fifty hard and brittle rings, which, with the exception of those forming the head and tail, differ but slightly from each other. Every segment supports two pairs of minute feet, arising close to the mesial line upon the under or ventral surface; but these feet, although distinctly articulated (fig. 139, c, p), are as yet extremely small in comparison with the bulk of the animal, and are evidently but mere rudiments of the jointed legs developed in more highly organized forms of homogangliate beings; the movements of the Julus are, consequently, very slow, and the creature seems rather to glide along the ground, supported on its numerous but almost invisible legs, than to walk. When at rest, the body is rolled up in a spiral form (fig. 139, b), the feet being concealed in the concavity of the spire, and thus protected from injury.
Fig. 139. Julus terrestris. A, in the act of progression; B, the same rolled up in a spiral form; C, segments of the body magnified, showing the mode of attachment of the feet (i, p) on each side of the mesial line (r) of the abdomen.
(724). The mouth resembles in structure that of the larvae of some insects, and is furnished with a pair of stout horny jaws, moving horizontally, and provided at their cutting edges with sharp denticulations, so as to render them effective instruments in dividing the fibres of rotten wood, or the roots and leaves of vegetable substances usually employed as food; and the alimentary canal, which is straight and very capacious, is generally found filled with materials of this description.