Peripatus has numerous tracheal pits; in other tracheate Arthropoda more or fewer of the somites possess each a pair of stigmata, the head alone excepted, on which they are very rarely present. It has been suggested that the tracheae represent modified glands (Moseley).

Excretory organs are present in Crustacea as (I) the two shell-glands, (2) the two antennary or green glands, (3) caeca of the mesenteron in Amphipoda, or of the proctodaeum in Squilla, whilst (4) in some instances the walls of the proctodaeum itself appear to be excretory. In tracheate Arthropoda the organs in question are tubular outgrowths of the proctodaeum known as Malpighian tubes, varying in number, shape, etc. Uric acid appears to be constantlv found amon? the excretorv products 1.

1 The coxal glands of Arachnida are possibly aborted excretory organs.

Arthropoda are very rarely hermaphrodite. The male is generally distinguished from the female by differences of size, colour, by the large development of special sense organs, or of copulatory and prehensile organs. The form of the testis and ovary is subject to much variation. The glands are typically paired, but the members of the pair may be fused. The ducts are also paired, but may fuse distally, and the sexual apertures are accordingly paired or single. They are placed in the thorax or abdomen, and are sometimes terminal. Accessory organs are frequently present, but vary in the different classes. The spermatozoa are commonly united into spermatophores by the secretion of the walls of the vasa deferentia, or of special glands. The ova vary in size. Secondary yolk is generally present, and is placed centrally (centro-lecithal ova). When the ovum segments, fission is consequently on the centro-lecithal type, and either regular, unequal, or superficial. But there are exceptions. Regular segmentation, with completely separate blastomeres, occurs in Podura among Insecta, and in the lower Crustacea, rarely in the higher.

Telo-lecithal ova, with partial segmentation, are found in some Crustacea (parasitic Copepoda, Isopoda, Mysis) and in the Arachnid Scorpio, but it is possible that they are modified from the centro-lecithal type. In development, certain differences are observable between the Crustacea and tracheate Arthropoda, which make it possible that they may form two distinct phyla, or branches of a phylum which separated at a very early period (?). In the Crustacea the archenteron is usually and primitively invaginate; the meso-blast originates from the wall of the invagination, and forms a layer between the epi- and hypo-blast; the proctodaeum is formed before the stomodaeum, and these two parts are usually of great relative length as compared with the mesenteron. In tracheate Arthropoda the archenteron is not invaginated, and the mesenteron is derived from the yolk-cells; the mesoblast developes as a median thickening of the ventral plate, which divides into two bands. These bands in Spiders and Myriapoda, and perhaps in Insecta, divide into somites, with cavities continued into the limbs. The stomodaeum developes before the proctodaeum, and the mesenteron is relatively large. The proctodaeum gives origin to the Malpighian tubes.

In the tracheate Arthropoda and higher Crustacea, a thickening of the blastoderm gives rise to a ventral plate, by the growth of which the body is subsequently formed. In the Insecta, and perhaps Myriapoda, this plate is marked by a median longitudinal mesoblastic groove, or by a keel-like thickening in Spiders. Groove and thickening alike are generally termed 'primitive streak,' and, like the structure so-called in Vertebrata, are probably the remnants of a blastopore.

Parthenogenesis occurs in some Insecta and Crustacea, and leads to a form of Alternation of Generations known as Heterogamy. Sexual dimorphism is very common in the two classes named. Some Crustacea possess the power of casting off limbs; all reproduce them when injured. A similar reproduction has been observed in some Insecta, Myriapoda, and Spiders. Mimicry of colour and shape is frequent, and in some Crustacea the expansion and contraction of pigment cells (chromatophores) may assimilate the tint of the organism to its surroundings. The life-history of a Crustacean is primarily, of an Insect secondarily, connected with a metamorphosis. In the higher Crustacea the metamorphosis becomes restricted or lost, in the higher Insecta more pronounced. There are but slight indications of similar changes in the other classes. Degraded forms, with regressive metamorphosis, are found e.g. in many parasitic Copepoda among Crustacea, Linguatulina among Arachnida.

There are five classes of Arthropoda usually recognised, the Insecta s. Hexapoda, the Myriapoda, the Protracheata, the Arachnida, and Crustacea, to which must be certainly added, as an independent group, the Pycnogo-nidae s. Pantopoda. The Insecta may perhaps have some relationship with the Myriapoda through the genus Scolopendrella. The class Protracheata, represented by the genus Peripatus, stands quite alone. But the three classes first-named agree together, and differ from the two last in having praeoral cephalic processes or antennae. The praeorally placed appendages of Arachnida and Crustacea appear to be postoral limbs, which have acquired a secondary praeoral position. It has been proposed in consequence to unite them under the common term Acerata. The majority of Arachnida are tracheate, and by some authorities the Insecta, Myriapoda, Protracheata, and Arachnida are therefore grouped together as Tracheata. But it does not appear certain that the tracheae of the four classes in question are homologous structures, and the Arachnida, as given below, include certain branchiate forms, viz.

Limulus, and the extinct Eurypterina and Trilobita. The young Limulus has a singular resemblance to the extinct Prestwichia. Many Trilobita have a similar larval form, and it is possible that the Scorpions and Eurypterina may be traced back to the same type. There are however certain sub-groups the position of which is exceedingly doubtful. These are the Linguatulina, the Tardigrada, and Acarina. They are generally classed among Arachnida, and are retained in that position here. It is possible, however, that they may have to be separated, and this is especially the case with the Acarina, which are placed apart by Haller in a Class, Acaroidea.

There are many interesting points relative to the connection of the classes of Arthropoda and certain of their sub-groups which cannot be discussed in a short compass. Reference must be made to the authorities cited below.

Haller gives the following characters for the class Acaroidea (Z. A. iv. 1881, p. 386): "Wingless Arthropoda, usually of small size, respiring partly through the integument, partly by tracheae. Cephalothorax and abdomen as a rule indistinguishably fused, seldom clearly separate, the line of division being usually indicated by a furrow. Each principal division of the body provided with two well-developed pairs of limbs. There are three pairs of jaws with a rudimentary upper lip and large under lip. The first pair of maxillae palpate, the second pair often rudimentary. The larva is hexapodous, the post-embryonal development frequently interrupted by a Deutovum stage.' Haller makes two orders, (1) 'Acarina atracheata, Mites devoid of tracheae, respiring through the integument;' and (2) 'A. tracheata, Mites provided with tracheae at least in the adult condition.'

It remains to be seen whether Haller's conclusions meet with acceptance and confirmation. The cuticular membranes cast off by some Mites in the course of development, and characteristic of the Deutovum and Tritovum stages, are by no means always developed. But if the mouth-parts really consist of three pairs, mandibles, first and second maxillae, and a palpate lower lip which represents a fourth pair of metamorphosed appendages, together with four pairs of limbs, then a Mite possesses two pairs of appendages not present in the adult Spider. But it should be borne in mind that there are in Spiders aborted embryonic limbs behind the last pair of legs, and the Scorpion actually retains two additional pairs in the genital operculum and pectinate appendages. See table, pp. 174-5. The Acarina might be regarded as a group of Arachnida with two additional pairs of limbs converted into mouth-parts, just as in the branchiate Arachnida there are five pairs of limbs in relation with the mouth in Limulus, four pairs in Eurypterina and Trilobita. Oudemans states that the fourth pair of ambulatory limbs is intercalated in the Oribatidae, between the first and second pairs of limbs of the hexapod larva.

Such a fact rather points to the conclusion that the hexapod stage has been secondarily acquired by the temporary suppression of a somite.

Affinities of Arthropoda, Kingsley, Q. J. M. xxv. 1885, p. 556 et seqq.; Oudemans, Tijdschrift der Nederland. Dierk. Vereen. (2), i. 1885, p. 37.

Phylogeny of Arthropoda, Balfour, Comp. Embryology, i. p. 451.

Acaroidea, Haller, Z. A. iv. 1881, p. 380.

Eyes of Arthropoda, Patten, Mitth. Zool. Stat. Naples, vi. 1886; Ray Lankester and A. G. Bourne, Q. J. M. xxiii. 1883.