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.
In this course the fluid may afford the materials of nourishment to the ova as they travel from one limb to another of the looped organ.
* Phil. Trans. 1858.
(595). The organ itself may be described as a looped tube, the limbs of which, returning upon themselves, and one invaginating the other, open externally at the same point. In order to examine further these delicate organs, they must be cut out, as closely to the abdominal surface as possible, by means of a curved pair of scissors. They should then be floated in water on a glass slide, covered by a thin slip, and placed under the microscope. If they have been successfully dissected out, it will be seen that one limb is broader and more transparent than the other: in the former limb the ova are merely pellucid cells, consisting exclusively of the germinal vesicle and germinal spot; in the latter limb the ova have acquired their full complement of vitellus. At this stage they exhibit a flattened ovoid form. This fact affords a demonstration as to the course taken by the ova in their passage from the ovary, properly so called, towards the exterior. The seat of the true ovary, or that part which is endowed with the special power of ovogenesis, is marked by the plexus of vessels. In relation to this special centre, all the rest of the organ may be characterized as the oviduct; but it is also a vitellarium.
It follows therefore that, in the Leech tribe, the ova at no time reach the cavity of the body: this may be one reason why, in this family, the perigastric cavity is so contracted, and the contained fluid so reduced in volume.
The species belonging to the second division of those Annelidans which possess no external organs of respiration are easily distinguishable from the suctorial worms by the different construction of their instruments of locomotion. They live in general beneath the surface of the ground, either perforating the soil in all directions, as the Earthworms (Lumbrici), or burying themselves in the mud upon the sea-shore or of freshwater streams, where many of them, called Naides (Nais, Linn.), live a semi-aquatic life. In conformity with such habits, their entire structure is adapted to a subterranean existence, and their bodies are so organized as to enable them to burrow with facility through the dense and unyielding materials wherein they are usually found. Whoever has attentively watched the operations of an Earthworm when busied in burying itself in the earth, must have been struck with the seeming disproportion between the laborious employment in which it is perpetually engaged, and the means provided for enabling it to overcome difficulties apparently insurmountable by any animal unless provided with limbs of extraordinary construction and possessed of enormous muscular power.
In the Mole and the burrowing Cricket we at once recognize, in the immense development of the anterior legs, a provision for digging, admirably adapted to their subterranean habits, and calculated to throw aside with facility the earth through which they work their way; but in the worms before us, deprived as they appear to be of all external members - feeble and sluggish even to a proverb, - where are we to look for that mechanism whereby they are enabled to perforate the surface of the ground, and to make for themselves, in the hard and trodden mould, the pathways that they traverse with such astonishing facility and quickness?
(597). The structure of the outer fleshy integument of the Earthworm resembles in every respect that of the Leech, already described, both in the annular arrangement apparent externally, and the disposition of the muscular strata. The suctorial disks, however, that in the Leech formed such important instruments of progression, are here totally wanting; and the annular segments of the body, as they approach the anterior extremity, become gradually diminished in size, so as to terminate, when the worm is fully stretched out, in a fine point, near the apex of which is the opening of the mouth. But there is another circumstance wherein the external anatomy of the terricolous Annelidans differs materially from what we have seen in the suctorial Abranchia: in the latter, the tegumentary segments were quite naked upon their outer surface; but in the Lumbrici, of which we are now speaking, every ring, when examined attentively, is found to support a series of sharp retractile spines or prickles; these, indeed, are so minute in the Earthworm, that, on passing the hand along the body from the head backwards, their presence is scarcely to be detected by the touch, but they are easily felt by nibbing the animal in the opposite direction, - a circumstance arising from their hooked form, and from their points being all turned towards the tail.
These differences between the external structure of the suctorial and setigerous Abranchia, minute and trivial as they might seem to a superficial observer, are, however, all that are required to convert an aquatic animal into one adapted to a subterranean residence, as will be evident to any one who observes carefully the manner in which the Earthworm bores its way through the soil. The attenuated rings in the neighbourhood of the mouth are first insinuated between the particles of the earth, which, from their conical shape, they penetrate like a sharp wedge; in this position they are firmly retained by the numerous recurved spines appended to the different segments: the hinder parts of the body are then drawn forwards by a longitudinal contraction of the whole animal - a movement that not only prepares the creature for advancing further into the soil, but, by swelling out the anterior segments, forcibly dilates the passage into which the head had been already thrust; the spines upon the hinder rings then take a firm hold upon the sides of the hole thus formed, and, preventing any retrograde movement, the head is again forced forward through the yielding mould, so that, by a repetition of the process, the animal is able to advance with the greatest apparent ease through substances which it would at first seem utterly impossible for so helpless a being to penetrate.