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.
(725). In most points of their internal organization, the Myriapoda resemble insects; and we should only anticipate the observations that will be more conveniently made hereafter did we enter into any minute description of their anatomy: we shall therefore, in this place, simply confine ourselves to the notice of those peculiarities observable in the animals under consideration whereby they are distinguished from insects and entitled to rank as a distinct class. We have seen that, in such of the Annelida as have been most carefully investigated, the orifices of the sexual organs are situated near the anterior part of the body, not, as is invariably the case among insects, at the caudal extremity: in this particular the Julidoe still present analogies with the red-blooded worms; for in them the external openings of the male parts are situated immediately behind the base of the seventh pair of legs, and are found to be placed upon minute mammillary protuberances, which are each furnished with a sort of hooked scale, adapted to hold the female during the process of impregnation.
(726). In the female also, the sexual orifices are advanced very far forward, being situated in the vicinity of the head, between the first and second segments; the sexes, however, as in insects, are perfectly distinct, and the conformation of the internal organs coincides with that type of structure which is common to the insect orders.
(727). The male generative organs of Julus are two elongated and partially convoluted tubes, placed side by side beneath the alimentary canal. The excretory ducts, or terminations of these tubes, run towards the anterior part of the body, where they terminate in two minute in-tromittent organs, situated at the under surface of the seventh segment, immediately behind the seventh pair of legs. As they pass backwards, the secerning tubes, or testes, gradually separate from each other, and have developed from their sides, at short distances from each other, numerous small glandular caeca, which doubtless constitute the secreting portions of the apparatus, or proper testes. The two efferent ducts, whereby the secretion of these caeca is conveyed out of the body, intercommunicate freely by means of short transverse canals, and, from the sacculated appearance that they present towards their termination, appear likewise to perform the office of reservoirs for the seminal fluid.
(728). In the female Julus, the organs of reproduction are as simple in their structure as those of the male. They consist of a single elongated bag or oviduct, covered on its exterior surface with a very great number of ovisacs or caeca of various sizes, each of which secretes but a single ovum. This oviduct extends backwards beneath the alimentary canal from the vaginal outlet, which is double, and situated in the fourth segment of the body, behind the second pair of legs. In the pregnant female the oviduct appears smooth externally, being distended with the ova that have passed into it from the ovisacs where they were formed, and which are retained in readiness to be deposited immediately after intercourse with the male.
(729). The ova, when fully developed, are found to present all the structures belonging to a perfectly formed egg, - the yelk, the germinal vesicle with its macula, the membrana vitelli, the albumen, and likewise the shell, lined by the membrana externa, or chorion, being all distinctly recognizable.
(730). Another important distinction between these animals and insects properly so called, is met with in the mode of their growth and development. Insects (as we shall more fully explain hereafter) undergo a more or less complete change in their outward form as they advance through several preparatory stages to their mature state: during the progress of these changes, that constitute what is usually called the metamorphosis of insects, they are invariably unable to perpetuate their species; and it is only in their last or perfect condition, which is ordinarily of very short duration, that the sexual organs attain their perfect development and are fit for reproduction. In this state all true insects have six legs, which is one of the most important characters of the class. The Myriapoda likewise undergo several changes of form as they advance to maturity; but these changes principally consist in the repeated acquisition of additional legs; so that in their perfect condition, instead of the limited number of six legs met with in insects, these organs have become extremely numerous.
The progress of these transitions from their immature to their fully-developed state has been well observed by De Geer* and Savi1; and the result of their observations is here given, in order that the reader may compare the different steps of the process with what we shall afterwards meet with in the more highly organized Articulata.
(731). The eggs (fig. 140, a), which are very minute, are deposited in the earth or vegetable mould in which the Julus is usually met with. When first hatched, the young Myriapod is of course exceedingly diminutive; at that period it resembles a microscopic kidney-bean, and is completely destitute of legs or other external organs. After a few days the embryo Julus changes its skin, and, throwing off its first investment, appears divided into distinct segments, and furnished with a head, a pair of simple eyes, a pair of antennae, and six jointed legs attached to the anterior rings of the body (fig. 140, b, c.) Some days subsequent to its-first moult, the skin is again cast, and the millepede, acquiring larger dimensions, is seen to possess seven pairs of ambulatory extremities, which are, however, still placed only upon the anterior segments (fig. 140, D.) When twenty-eight days old, they again throw off their outward covering, and assume, for the first time, their adult form: they then consist of twenty-two rings, and have twenty-six pairs of feet; but, of these, only the eighteen anterior pairs are used in progression. At the fourth moult the number of legs is increased to thirty-six pairs; and at the fifth, at which time the body becomes composed of thirty segments, there are forty-three pairs of locomotive organs. At last, in the adult state, the male has thirty-nine and the female sixty-four rings developed; but it is not until two years after this period that the sexual organs appear and the animals become capable of reproduction.