« AnteriorContinuar »
of its parts. The skin is more or less hardened or roughened by means of lime deposited in it. There is a peculiar system of tubes or “water-vessels," which contain water, usually communicate with the exterior, and are generally employed in locomotion. There is a distinct nervous system consisting of a ring-like cord surrounding the mouth, and sending off branches in a radiating manner. These characters distinguish the class of the Echinodermata as a whole.
The class Scolecida (Greek, skolex, a worm) includes chiefly the various worm-like animals which live parasitically in the interior of other animals. Besides these there is a number of nearly related forms, which lead a free existence, together with the singular group of the Wheelanimalcules. One of these last we shall select as an example of this class, not as being by any means a typical example, but as not presenting certain disadvantages under which other more characteristic forms labour.
If we take one of the free-living Wheel-animalcules, such as Eosphora (fig. 8), we find that we have to deal with a microscopic, translucent, little creature, which is to be detected swimming about actively in fresh water. The name of “Wheel-animalcule” refers to one of its most prominent peculiarities-namely, that the front end of the body is in the form of a disc surrounded by a fringe of little vibrating hair-like processes (“cilia”). When these latter are in active motion, the whole head looks as if it were rotating rapidly, like a wheel. By means of this disc the animal not only drives itself through the water, but also sets up currents which bring floating particles of food to the mouth. At the hinder end of the body there is a little pair of pincers, composed
of two little diverging “toes,” and by means of these the animal can at will moor itself to the stems of aquatic plants. We can also observe that the integument is to a certain extent ringed or transversely wrinkled, though not nearly in such a marked manner as in the true Worms.
As before remarked, the animal is nearly transparent, and its internal anatomy can thus be readily studied. On one side of the locomotive wheel is placed the opening of the mouth (fig. 9, b), to which a depression in the disc conducts. The upper portion of the gullet (c) is much dilated, and contains a complicated series of horny jaws. There is a well-developed stomach (d), and the intestine opens into a chamber, with which the “water-ves
sels” also cominunicate. Fig. 8.-Rotifera. Eosphora aurita, one of the Wheel-animalcules. En larged about 250 diameters. (After alluded to, are two tubes (9 Gosse).
g), which run along the sides of the body, and open behind into a contractile bladder (f). What the exact function of these vessels may be is uncertain, but they are supposed to be connected with the process of respiration.
There is no distinct heart, or true blood-system of vessels. The nervous system, however, is well developed, and consists of a little double nervous mass (h) situated
near the head, and carrying upon it the eye, in the form of a brightly-coloured spot.
Wheel-animalcules, such as here described, may be readily detected in most ponds, ditches, or slowrunning streams where waterplants grow abundantly. Though microscopic in their dimensions, they are interesting objects of study, owing to the facility with which their transparent skin allows their internal organs to be seen.
RECAPITULATION OF ESSENTIAL CHARACTERS.—The body does not exhibit definite segmentation, nor does it carry upon its sides symmetrically disposed appendages.
There is no distinct heart nor blood-vessels, but there is a remarkable set of vessels which usually communicate with the Fig. 9.—Diagram of the anaexterior. The nervous system has
vous masses. These characters distinguish the class of the Scolecida as a whole.
tomy of a Wheel-animalcule. a Depression in the wheel-organ leading to the mouth (b); c Dilated upper portion of the gullet, with the horny jaws; d Stomach; e Chamber into which the intestine opens; gg Water - vessels, opening into contractile chamber (f); h Nervous system.
THE Ringed Worms, or “ Annelides,” comprise such animals as the Leeches, Earth-worms, Water-worms, Tubeworms, and Sand-worms, and derive their name from the fact that the integument or skin is markedly ringed or thrown into transverse folds (Latin, annulus, a ring). As an example of this class, we may select the Common Medicinal Leech (Sanguisuga officinalis).
The Leech (fig. 10, a) is of an elongated, cylindrical, worm-like form, tapering towards its head, but capable of
greatly contracting or lengthening itself at will. Its skin is quite soft and flexible, slimy to the touch, and very markedly ringed or transversely folded, the total number of rings being about one hundred. At each extremity of the body it shows a little sucking
by means of which it can move actively about. The hinder sucker (ps) is distinctly constricted off from the body by a kind of neck, is not perforated by any aperture, and is provided with an even circular margin. The front sucker is of an elongated form (as), is really formed mainly out of the
elongated upper lip, and is perFig. 10. - a The Medicinale
forated by the opening of the Leech (Sanguisuga officinalis), natural size ; 6 An- mouth. When moving, the Leech terior extremity of the same magnified.showing the fixes its hinder sucker. and sucker and triradiate jaws; stretches out its head till it meets c One of the jaws detached showing the semicircular with some solid object; it then toothed
detaches the hinder sucker, and brings this forward till it adheres close beside the spot where the head has fixed itself. Then, detaching the head, it again extends itself to seek another point of attachment further on. By a repetition of this process, the Leech can travel with considerable rapidity; but it also swims well by a serpentine bending of the body.
The body of the Leech is absolutely destitute of limbs
or appendages of any kind, and locomotion is entirely effected in the manner just alluded to.
Fig. 11.-A, Diagram of the Leech, showing the nervous system, and the ten
eyes placed on the top of the head. B, The leech dissected to show the alimentary canal (c), and the so-called “respiratory sacs” (
rr); as Anterior sucker; ps Hinder sucker; nn Nervous system; h Head, carrying the eye-spots.
The mouth opens at the bottom of the front sucker, and is provided with three crescentic jaws (fig. 10, 6 and c),