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.
(738). On again casting its skin, the new segments of the body produced at the former change, from the eighth to the twelfth inclusive (fig. 141, e, 8-12), are become of the same size as the original ones, and each has developed from it two additional pairs of legs, so that the whole number becomes increased to thirty-four; and thus at each change of skin the number of new segments and of additional legs is increased, by development from the germinal membrane, until the full complement is acquired.
In the second family of Myriapoda we have a very striking illustration of the manner in which the development of the nervous centres proceeds step by step with that of the external limbs. The slow-moving Julidae possess, in their rudimentary feet, organs adapted to their condition; and their feeble powers of locomotion are in relation with their vegetable diet and retiring habits. But in the predaceous and carnivorous Scolopendra (fig. 142), which, although it lurks in the same hiding-places as the Julus, obtains its food by pursuing and devouring insects, far greater activity is indispensable; and accordingly we find the segments of the body, and the extremities appended to them, exhibiting a perfection of structure adapted to greater vivacity and more energetic movements.
(740). This is at once evident upon a mere inspection of their outward form: the individual segments composing the animal are much increased in their proportionate dimensions, and, instead of being cylindrical, each division of the body is flattened and presents a quadrangular outline. In order to give greater flexibility to the animal, instead of the semicrustaceous hard substance which forms the rings of the Julus, the integument is here composed of a tough and horny substance, forming two firm plates, one covering the back, the other the ventral aspect of the segment, while all the lateral part is only incased in a flexible coriaceous membrane, with which the individual rings are likewise joined together. Such an external skeleton is obviously calculated to give the greatest possible freedom of motion, and thus to enable the Scolopendra to wind its way with serpent-like pliancy through the tortuous passages wherein it seeks its prey.
(741). The ventral chain of ganglia belonging to the nervous system presents a series of nervous centres of dimensions proportioned to the increased bulk of the segments in which they are lodged, and becomes thus fitted to direct the movements of more perfect limbs. The legs, therefore, as a necessary consequence, are now proportionately powerful, divided into distinct joints, and provided with muscles calculated to bestow on them that activity essential to the pursuit and capture of active prey. Thus, then, by a simple concentration of the nervous masses composing the abdominal chain of ganglia, we have the slow-moving and worm-like Julus (which we have seen to be, in consequence of its feebleness, restricted to live upon roots and dead substances) converted into the carnivorous and powerful Scolojoendra, well able to wage successful war with the strongest of the insect tribes, and not unfre-quently formidable, from its size, even to man himself.
Fig. 142. Scolopendra.
(742). The mouth of the Scolopendra is a terrible instrument of destruction, being provided not only with horny jaws resembling those of insects, hereafter to be described, but armed with a tremendous pair of massive and curved fangs ending in sharp points, and perforated near their termination by a minute aperture, through which a poisonous fluid is most probably instilled into the wound inflicted by them. It is to this structure that the serious consequences which in hot climates not unfrequently result from the bite of one of these animals must no doubt be attributed.
(743). In their internal anatomy the Scolopendridce resemble insects even more nearly than the Julus. The alimentary canal is straight and intestiniform, but of much smaller diameter than that of the vegetable-eating Myriapoda. It presents an oesophagus and a small muscular gizzard; but there is no perceptible division into stomach and intestine. The respiratory and circulating systems, so far as they are understood, seem to correspond with what we shall afterwards find to exist in the larvae of insects.
(744). In the Seolojpendridae, as we learn from the researches of Mr. Newport*, the heart is enclosed in a distinct membranous covering, which may be regarded as a true pericardium, consisting of a loose delicate membrane, between which and the sides of each chamber of the heart there is a slight interspace. The heart itself is a long pulsating organ, corresponding in its general structure and position with the dorsal vessel of insects; it is situated immediately beneath the integuments, and runs along the mesial line of the dorsal region of the body, consisting of a series of chambers, twenty-one in number, that communicate with each other and extend through the entire length of the animal from the tail to the cephalic segment.
(745). The minute structure of the heart is exceedingly interesting. This organ is composed of two distinct contractile tunics, one external and the other internal, each being covered by its proper serous membrane. The external tunic is a very thick muscular layer, the fibres of which are loosely interwoven with each other. The internal tunic is composed of two sets of muscular fibres, of which the inner stratum is disposed longitudinally, while the external one is formed of numerous short, broad, transverse muscular bands, very much resembling in appearance the cartilaginous rings of the trachea in vertebrated animals. They do not completely encircle the longitudinal ones, but pass only halfway round on each side, leaving a space between those of the two sides, both upon the upper and under surface.
* Phil. Trans. 1843.
(746). Prom each compartment of the heart proceed the systemic arteries, which supply nearly the whole of the blood to the viscera and lateral portions of each segment. The anterior pair of these systemic arteries, however, instead of being distributed like the rest, form a vascular collar, which, after surrounding the oesophageal tube (to which, and to the different parts belonging to the cephalic segment, it gives off numerous branches), unites beneath the oesophagus to form the great supra-ganglionic vessel or aortic trunk,extendingbackwards along the middle line of the body, immediately above the centres of the nervous system (which it supplies plentifully with blood), as far as the terminal ganglion in the last segment, giving off in its course numerous arterial canals, which ramify extensively in the surrounding structures. The return of the blood from the various viscera to the dorsal vessel is effected, as in insects, by lacunar or interstitial channels, as will be explained in the next chapter.
(747). In the position and arrangement of the sexual organs, the Scolopendridae complete the transition between the Anne-lidans and Insects properly so called; for while in Julus we have found them still occupying the anterior part of the body as in the former class, in Scolopendra they are removed to the tail. The structure of the male organs (fig. 143,2) is remarkable. The testes are seven in number; and on opening the posterior segments of the animal, they are found closely packed in parallel lines: each testis is composed of two fusiform parts precisely similar to each other; and from both ends of every one of these, which are hollow, arises a narrow duct; so that there are fourteen pairs of ducts arising from the fourteen secreting organs. The ducts all end in a common canal, which gradually becomes enlarged and tortuous, and terminates by a distinct aperture in the vicinity of the anus. Just prior to its termination, the common ejaculatory duct communicates with five accessory glands (fig. 143, 2, c,dd, e e), four of which are intimately united until unravelled, while the fifth is a simple caecum of considerable length*.
Fig. 143. 1. Female, and 2. Male generative system of Scolopendra.
(748). The ovarian system of the female Scolopendra is a single tube (fig. 143, l), without secondary ramifications, but receiving near its termination the ducts of accessory glands, as represented in the figure.
(749). Some Scolopendrae (S. phosphorea) emit, in the dark, a strong phosphorescent light; and one species (S. electrica) is able to give a powerful electrical shock to the hand of the person who inadvertently seizes it.