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.
Careful experiments, made to ascertain how far the statements of former authors upon this subject might he substantiated, prove that the assertion is not entirely without foundation, although by no means to the extent indicated in their writings. It would, indeed, be easily credited that the removal of the hinder part of the body of an Earthworm would not necessarily destroy the anterior portion, since no organs absolutely essential to existence are removed by the operation, and even the course of the circulating fluids would not be materially interrupted by the mutilation; but that the hinder moiety should be able to reproduce the mouth, gizzard, and stomach, the complicated apparatus of moniliform vessels, and the sexual organs, contained in the anterior segments, could scarcely be deemed possible; and the assertion has been satisfactorily disproved by actual observation. On cutting an Earthworm in two, the anterior portion is found, in fact, generally to survive; and the wound caused by the operation, becoming gradually constricted, is soon converted into an anal orifice, rendering the animal again complete in all parts necessary for its existence.
This, however, is by no means the case with the posterior portion; for, although it will exhibit for a very long period indications of vitality, no signs of reproduction have been witnessed, and it invariably perishes.
(627). Nevertheless, although it is thus proved that the Earthworm cannot be multiplied by mechanical division, it is stated to be able to reproduce small portions of its body the removal of which does not implicate organs essential to life. In the experiments of M. Duges, for example, it was found that four, or even eight of the anterior rings might be cut off with impunity, although the cephalic pair of ganglia, the mouth, and a part of the oesophagus were necessarily taken away. In worms thus mutilated, after the lapse of from ten to thirty days a conical vascular protuberance was observed to sprout from the bottom of the wound; and in eight or ten days later this new part had become so far developed, that not only all the lost rings were apparent, but even the upper lip and mouth had assumed their normal form, and the animal again began to eat and bury itself in the earth.
(628). The experiment of artificially bisecting the body of an Earthworm, replacing the divided halves with care again in their native habitats, invariably, in the hands of Dr. Williams, led to the following results: - The cephalic half, by this division of the body, does not lose the power of locomotion. In a few days after the operation, it begins to grow less active and vigorous in its movements, and the annulus at the point of division begins to contract and wither; in the course of a few more hours it dies - it mortifies away. This process of dissolution creeps, in the direction of the head, from one segmental ring of the body to another, until, finally, the cephalic remnant ceases to manifest any signs of life.
Fig. 115. Eggs of the Earthworm.
(629). The tail-half immediately loses the power of advancing; it writhes on one spot, and that only in contact with some external body; its motions become excited, not voluntary; it never re-acquires the power of swallowing earth. The process of decay begins much sooner than in the cephalic half, and extends in the direction of the tail, implicating one ring after another rapidly, until the whole perishes.
(630). The little lively Naicles, although terricolous in their habits like the Earthworm, are very dissimilar in organization.
(631). In Nais filiformis, so abundant in the freshwater pools of this country, the anatomist is presented with a favourable opportunity of resolving the problem of the circulation. A living specimen placed between two slips of glass, from the perfect transparency of the integuments, will exhibit to the eye in a perfect manner all the circulating movements both of the vessels and of the blood. In Nais, the large dorsal vessel (fig. 116, a) is first seen travelling wavingly along the dorsum of the intestine as far as the heart, which corresponds in situation with the intestinal end of the oesophagus. This vessel is enveloped by the glandular peritoneal layer of the intestine, while the coats of the ventral vessel are clear and transparent: the dorsal vessel is endowed with parietes of greater strength and density than the ventral. Each of these vessels dilates into a fusiform heart (fig. 116, a', b'), situated on either side of the oesophagus. These hearts, which are joined together by transverse vessels, pulsate alternately and with exact regularity. In the dorsal vessel the blood moves forwards from the tail as far as the dorsal heart; thence it descends into the ventral heart, by which it is now propelled, chiefly in a backward direction, partly through the main ventral trunk, and partly through the inferior intestinal. The other portion of the blood conveyed by the great dorsal vessel into the ventral heart (b') passes forwards as far as the head, where its moving power is again reinforced by a cardiac dilatation, which now impels the current from before backwards through a superior oesophageal trunk into the dorsal heart (a'), by which organ the blood received from the region of the oesophagus and coming from the head, as well as that received from the great dorsal and coming from the tail, is urged downwards into the ventral heart, and thence, chiefly in the direction of the tail, through the ventral and intestinal trunks (f,e); this latter therefore is the true systemic heart.
At the oesophageal end of the body, the two primary trunks, dorsal and ventral, are connected together by means of a remarkable class of vessels (g g g), which in this region proceed at successive points from the dorsal oesophageal, and which may be traced in long coils, without division of the vessel, floating in the fluid of the peritoneal cavity. Posteriorly to the heart-centre these vessels emanate from the dorsal intestinal, and correspond precisely with those branches from the same vessel which in Arenicola piscatorum proceed to supply the branchial arbuscles. In Nais, therefore, partly from this analogy, but chiefly from their anatomical relations, bathed by and floating in the chylaqueous contents of the peritoneal cavity, the physiologist can experience no difficulty in dedicating these coiled vessels to uses very definite. First, it cannot be doubted that they absorb from this fluid the elements by which the blood-proper is formed and replenished; and secondly, it is in the strongest-degree probable that the true blood is in great part aerated through the agency of these vessels upon the gaseous elements contained in the peritoneal fluid. They constitute the special branchial system (internal branchiae), while they discharge incidentally an absorbent function.