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.
(566). The lowest class of articulated animals comprehends an extensive series of creatures generally grouped together under the common name of Worms. In the outward form of their bodies many of them resemble some of the more perfect Entozoa, and we need not therefore be surprised that, in ordinary language, they are frequently confounded together. But whatever may be the similarity in outward appearance between the more perfect intestinal worms and the animals belonging to the class upon the consideration of which we are now entering, the examination of their anatomical structure will at once show that they differ widely from each other, and have thus been properly separated by a considerable interval in all the more modern systems of zoological arrangement.
(567). The principal characters which serve to distinguish the Annelida from other forms of the animal world are readily appreciated, and, when once pointed out, will be found sufficient for the guidance of the most superficial observer. The body is always considerably elongated, and composed of a succession of rings or segments that, with the exception of the first and last, scarcely differ from each other except in size. Each ring is generally found to be furnished with a set of short spines or setae, calculated to assist in locomotion; but in no instance are these animals provided with articulated legs. The first segment of the body, which may be called the head, contains the mouth, sometimes provided with a formidable apparatus of jaws; it is also generally furnished with eyes, and variously-shaped tentacula, apparently instruments of touch. The last segment also not unfrequently presents seti-form appendages, and occasionally a prehensile sucker, used as an organ of progression.
* Vide Brandt, Bemerkungen uber die Mundmagen- oder Eingeweidenerven der Evertebraten. Leipzig, 1835.
(568). Their blood is sometimes remarkable for its red colour, and circulates in a double system of arteries and veins; respiration is effected either in the general cavity of the body, or by means of arborescent tufts appended to various parts of their external surface; moreover they are almost all hermaphrodite, and generally require the congress of two individuals for mutual impregnation.
This order comprises two distinct tribes, that differ widely in their habits and external appearance: the first comprehends the Leeches (Annelida suctoria), distinguished by the existence of a prehensile sucker situated at each extremity; while in the second, instruments of attachment are totally wanting, the only external appendages to the body being a number of minute and almost imperceptible bristles, which project from the different segments and assist in progression: such are the Earthworms, etc. (Annelida terricola).
(570). The common Leech (Hirudo medicinalis) affords the most interesting example of a suctorial Annelid. The outward form of one of these animals is familiar to every one, and their general habits too well known to require more than a brief notice. The body is very extensible, and divided by a great number of transverse lines into numerous rings, apparent in the contracted state of the animal, but nearly imperceptible when the body is elongated. The skin is soft, being merely a thin cuticular pellicle separable by maceration; and the surface is lubricated by a copious secretion of mucus. Beneath the cuticle is a layer of coloured pigment, upon which the colours of the animal depend; but the cutis, or true skin, is so intimately connected with the muscular integument of the body, that its existence as a distinct tunic is scarcely demonstrable. The muscular coverings or walls of the body, which form a kind of contractile bag enclosing the viscera, are found, upon accurate dissection, to consist of three distinct strata of fibres running in different directions.
The outer layer is composed of circular bands, passing transversely; in the second the fibres assume a spiral arrangement, decussating each other; while the internal layer is made up of longitudinal muscles, extending from one end of the creature towards the opposite. Such an arrangement is evidently adequate to the production of all needful movements, and capable of giving rise to all the motions connected with the elongation, contraction, or lateral inflexions of the body used in progression.
(571). At each extremity of the animal, the muscular coat expands into a flattened fleshy disk, composed of circular and radiating fasciculi, which, when applied to a smooth surface, perform the office of suckers, and thus become important instruments of prehension. There are no vestiges of external limbs; nevertheless, with the simple mechanism above described, the Leech is able to crawl with considerable rapidity along the surface of subaquatic plants, or even to swim with much facility through the water. The first method of locomotion is accomplished by means of the terminal suckers. Supposing the posterior disk to be attached, the animal elongates its body to the utmost, and then fixes the sucker placed at the opposite extremity; this done, the hinder parts are drawn forward and again fixed, preparatory to a repetition of the process. In swimming, the whole body is elongated, and by some partial contractions of the muscular integument, not precisely understood, assumes the appearance of a flattened band; in this condition the Leech makes its way through the element it inhabits by successive undu-latory movements of the body, performed with much grace and elegance.
(572). The mouth of the Leech is an exceedingly complete apparatus, adapted not only to the destruction of minute aquatic animals that constitute its usual food, but, as is universally known, admirably fitted to extract blood from the higher animals; combining, in its operation, the offices both of the cupping-glass and the scarificator.
(573). The mouth is situated near the centre of the anterior sucker, so that the oral aperture is firmly applied to any surface upon which this part of the animal is fixed. Around the entrance of the oesophagus are disposed three minute cartilaginous teeth, imbedded in a strong circle of muscular fibres (fig. 106, a.) Each tooth has somewhat of a semicircular form, and, when accurately examined with a microscope, is found to have its free margin surmounted with minute denticulations (fig. 106, b), so as to resemble a small semicircular saw. On watching a leech attentively during the process of biting, the action of these teeth is at once evident; for, as the skin to which the sucker is adherent is rendered quite tense, the sharp serrated edges of the teeth are pressed firmly against it, and, a sawing movement being given to each cartilaginous piece by the strong contractions of the muscular fibres around the neck, these instruments soon pierce the cutis to a considerable depth and lay open the cutaneous vessels, whence the creature sucks the fluid which its instinct prompts it to seek after with so much voracity. The position of the teeth around the opening of the mouth, as represented in the subjoined figure (fig. 106, a), will at once explain the cause of the triradiate form of the incision that a leech-bite invariably exhibits.