Page:EB1911 - Volume 18.djvu/494

 internal or proximal ends of which are continuous with the fibres of the optic nerve. The ovary is unpaired and extends almost the entire length of the body beneath the alimentary canal. The oviducts are sometimes separate tubes (Lysiopetalum), sometimes confluent and divided just before terminating in the two orifices behind the base of the legs of the second pair (Julus). The testes and seminal ducts occupy the same position and extent as the ovary and oviducts. The ducts are sometimes coiled, sometimes divided, sometimes united. The two testes are sometimes united by transverse branches across the middle line, and are sometimes branched posteriorly. They bear short caecal diverticula in which the semen is developed. There, are no accessory glands associated with the generative organs; but in some forms, e.g. Polyxenus, there is a pair of receptacula seminis extending backwards alongside the ovary and opening into the oviduct. The secondary sexual characters of the males are of great taxonomic importance. The seminal ducts, like the oviducts, open behind the legs of the second pair. Associated with them in the Limacomorpha (Glomeridesmus), there is a pair of very long retractile penes. In the Spirostreptoidea and Juloidea the penes are much shorter and have coalesced. Sometimes they are undeveloped (Spiroboloidea). In other cases, the Merochaeta, Oniscomorpha, &c., the ducts traverse the coxae of the legs of the second pair. But in all these groups, with the exception of the Oniscomorpha, semen is transferred from the genital orifices, with or without the aid of the penes, either into the first or second pairs of appendages of the seventh segment which are modified in various ways, and are termed phallopods. When the posterior legs are so modified the anterior are as a rule even more profoundly altered to form a protective sheath, or coleopod, for the phallopod; and as a further precaution the entire apparatus is usually withdrawn within the seventh segment. In the Oniscomorpha the semen is transferred into a pair of receptacles developed upon the coxae of the legs of the last pair, which are chelate. The male appendages that are modified in the above described ways are comprehensively spoken of as gonopods. Other secondary sexual characters, like the stridulating organs of the males of some Oniscomorpha, the suctorial pads on the legs of Spirostreptoidea, the development of angular processes upon the mandible or first tergal plate, or of fine ridges in the gnathochilarium—all of which are concerned in enabling the male to maintain a secure hold upon the female—are of great taxonomic use in distinguishing the genera and species.

The most important glands in the Diplopoda are the repugnatorial or stink-glands, which, except in the Oniscomorpha, Limacomorpha and Ascospermophora, open by pores upon the sides of more or fewer of the segments. They secrete a fluid with an unpleasant odour, breaking up in one case into cyanide of potassium, and are practically the only means of protection, apart from the hard exoskeleton, which Diplopods possess. In some millipedes silk glands also exist and open upon papillae upon the posterior border of the last tergal plate. They are found in the Ascospermophora, Stemmiuloidea and Proterospermophora, and are used for spinning nests for the eggs and protective cases for the young during exuviation.

Classification.—The existing members of the class Diplopoda may be classified as follows:—

Subclass

Diplopods with the soft integument strengthened by weakly chitinized sclerites and furnished above and on the head with transverse rows of short, stout, somewhat squamiform bristles; laterally, on each side of the principal segments, with a thick tuft of long bristles and with a large, silky, white tuft projecting backwards from the posterior extremity. Mandibles one-jointed. Behind them a pair of small, one-jointed maxillulae, attached to a median membranous “lingua.” Behind the “lingua” and maxillulae, a large, double, transverse plate with a long, external sclerite bearing distally in Polyxenus an inner short-lobate process and an outer long spiny palpiform branch. The latter, however, is absent in Lophoproctus. These sclerites probably represent the gnathochilarium of the Chilognatha, but the homology between the skeletal elements of the jaws in question is not clearly understood. It has been suggested that they represent two pairs of jaws, but embryological proof of this does not exist. Order Penicillata (＝Ancyrotricha). Head large, usually with lateral eyes. Antennae eight-jointed, attached near the middle of the front of the head. On the dorsal side of the body there are eleven segments, simple and compound. The first four of these bear one pair of legs each, the succeeding four two pairs of legs, the ninth segment one pair, making a total of thirteen pairs of legs. The tenth and eleventh or anal segment are legless. There is a narrow sternal area separating the bases of the legs of the two sides. There are no repugnatorial glands. In the male none of the legs are modified as gonopods, but the coxa of each of the legs of the second pair is furnished with a conical penis, which during copulation, it may be supposed, is inserted into the genital orifice of the female, which occupies a corresponding position in that sex. The young when first hatched has only three pairs of legs and five segments. The millipedes of this order are all of small size, measuring at most only a few millimetres in length. The best-known genera are Polyxenus and Lophoproctus, both of which occur in Europe. Other forms have been discovered in the West Indies, North and South America, and Ceylon; and it is probable that the group has an almost cosmopolitan range. They live under stones or the loosened bark of trees. The carboniferous fossil, Palaeocampa, is usually referred to this subclass.

Subclass

Diplopods with firmly chitinized exoskeleton, sometimes thickly, sometimes sparsely covered with short, simple hairs, but never decorated with tufts or rows of peculiarly modified bristles.