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.
(400). Although the existence of a nervous system in the Planarise has been doubted by some observers, the researches of M. de Quatrefages assure us of its presence in many species. It consists of two ganglions, more or less intimately united, which are situated in the mesial line near the anterior part of the body. This double ganglion, which may be called the brain, and which is sometimes visible to the naked eye, is lodged in a special lacuna or cavity, recognizable from its transparent outline, and is seen to give off nervous filaments in various directions to different parts of the body.
(401). M. Blanchard*, in dissecting a large individual belonging to this group, not only found the two cerebroid nervous centres, above alluded to, closely approximated, but observed that they gave origin to two long cords, which exhibited at regular intervals a series of minute ganglia, thus clearly approximating to the type of structure that characterizes the lower forms of the annulose worms.
(402). Many species of Planarise possess two red specks upon the anterior part of the body, which, as in other cases, have been unhesitatingly pronounced to be eyes, although their claim to such an appellation is not only unsubstantiated by any proofs derivable from their structure, but completely negatived by experiments, which go to prove that, in the pursuit of prey, no power of detecting the proximity of their food, by the exercise of sight, is possessed by any of them.
(403). The phenomena which have been observed, connected with the multiplication of the Planarise by division, are analogous to those which we have witnessed in other acrite animals; for it has been proved that, if an individual be cut to pieces, every portion continues to live and feel, from whatever part of the body it may be taken; and, what is not a little remarkable, each piece, even if it be the end of the tail, as soon as the first moment of pain and irritation has passed, begins to move in the same direction as that in which the entire animal was advancing, as if the body was actuated throughout by the same impulse; and, moreover, every division, even if it is not more than the eighth or tenth part of the creature, will become complete and perfect in all its organs.
* Sur 1'Organisation des Vers, Ann. des Sc. Nat. 1847.
(404). The mouth, in a few species of Planariae, is placed at the anterior extremity of the body, but generally 'it is found to occupy the middle part of the ventral surface. Its structure is quite peculiar, and admirably adapted to the exigences of the creature: it consists of a wide, trumpet-shaped proboscis (fig. 75, 3 & 4), which can be protruded at pleasure, and applied to the surface of such larvae or red-blooded worms as may come within reach, so as to suck from them the juices which they contain; or if the prey be small, such as animalcules and minute Crustacea, they are seized by it and conveyed into the digestive canals. The internal organs appropriated to nutrition resemble in all essential points those of theDistoma; they consist of a multitude of blind tubes, ramifying in the parenchyma of the body, which, when distended with coloured substances, are sufficiently distinct. The principal trunk (fig. 75, l), which communicates with the probosci-diform mouth, soon divides into three primary branches, one of which runs along the median line towards the anterior extremity, whilst the other two are directed backwards towards the tail. From these central canals secondary ones are given off, which permeate all parts of the creature. There is no anal aperture; so that, of course, the residue of digestion is expelled through the mouth; but the nature of the process by which defecation is thus effected is curious: the Planaria, slightly bending itself, is seen to pump up through its proboscis a quantity of water, with which all the branches of the alimentary ramifications are filled; the creature then contracts, and, forcibly ejecting the contained fluid, expeb with it all effete or useless matter.
(405). In the larger marine species, M. de Quatrefages* recognizes the existence of an internal general or visceral cavity, which he invariably found filled with a transparent fluid kept in continual agitation by the various movements of the animal. The flux and reflux of this fluid are rendered conspicuous by the movements hither and thither of numerous round diaphanous corpuscles which enter as morphotic elements into its composition and render indisputable its identity of character with the chylaqueous fluid of the more highly organized worms, in which the internal viscera are freely suspended, their parietes being merely kept in situ by delicate membranous frena.
Fig. 75. Structure of Planaria (after Duges.) 1. Ramiflcations of alimentary canal. 2. Vascular system. 3. Proboscis unfolded. 4. Represents a Planaria devouring a Nais, showing the action of the proboscis. 5. Generative system: a, male apparatus; b, female ditto. 6. Two Planariae in the act of copulation.
* Ann. des Sc. Nat. 1845.
(406). Besides the arborescent tubes in which digestion is accomplished, a rudimentary vascular system is distinctly visible, whereby the nutritive juices are dispersed through the system. This consists of a delicate network of vessels, arising from three large trunks, one placed in the centre of the dorsal aspect, and the other two running along the sides of the animal (fig. 75, 2).
(407). The Planariae are perfectly androgynous, as each individual possesses a distinct male and female generative system; but they are not, apparently, self-impregnating, seeing that the cooperation of two individuals has been found needful for the mutual fertilization of their ova. In every one of these animals two distinct apertures exist upon the ventral surface, at a little distance behind the root of the proboscis, the anterior of which gives issue to the male organ, while the posterior leads to the ovigerous or female parts.