In a polymeniscous eye a single lens-facet, a vitrella, and retinula constitute an 'element/ or the two latter, i.e. vitrella and retinula, an ommatidium. The vitreous cells retain the power of forming cuticle, since that structure is shed at each moult over the eye as elsewhere. In the higher Crustacea, in Scutigera among Myriapoda, and in Insecta, there are special optic ganglia in connection with the retinal layer, but external and posterior to it. The eye of Peripatus is formed on the Annelidan and Molluscan type, not the Arthropodan1. Auditory organs exist in the shape of special hairs in various regions of the body-surface (Araneidae, many Crustacea, ? Insecta), lodged also in the higher Crustacea in special depressions or sacs of the cuticle. They also exist as closed vesicles, containing one or more otoliths, in a few Crustacea and Insectan larvae. Olfactory and gustatory structures have been described. Tactile hairs connected with nerves occur, at least in Insecta.
The digestive tract consists of a stomodaeum and proctodaeum, both lined by cuticle and invaginated from the ectoderm. They are connected by a mesenteron developed from the archenteron. Salivary glands in connection with the stomodaeum occur in Myriapoda, Arachnida, and to a very limited extent in Crustacea. The corresponding glands in Insecta belong to the oral cavity, not to the stomodaeum proper. The epithelium of the mesenteron is often partly glandular (Insecta), or the external surface carries either villiform glands or larger caeca, which are much branched and of great size in the higher Arachnida and Crustacea.
The coelome contains the blood and amoeboid blood corpuscles. Circulation of the blood may depend solely in the animal's movements, or on rhythmic motions of the intestine (e.g. some Copepoda among Crustacea). A contractile heart, composed of a single chamber or many chambers, each with its pair of ostia, is generally present, and lies dorsally. The lips of the ostia form internal valves which prevent regurgitation. The heart is always open anteriorly, and, except in Insecta and Myriapoda, as a rule posteriorly also. The extent to which arterial capillary and venous channels are differentiated varies, but it is slight except in Myriapoda, and especially in Scorpio and Limulus, and the highest Crustacea. A pericardial sinus exists in the higher Crustacea, Arachnida, the Myriapoda and Insecta. In the three classes last named, it is dilated by a system of segmentally arranged alary muscles, which affect the heart only indirectly. The blood currents flow in constant directions.
1 The account in the text follows that given by Professor Lankester and Mr. A. G. Bourne. Patten has quite recently published some investigations on the compound polymeniscous eye of Crustacea and Insecta which, if confirmed, must considerably alter the conception formed of its structure. See Mitth. Zool. Stat. Naples, vi. 1886. His chief results are the following: (1) To each corneal facet coincides a group of two hypodermic cells, sometimes very difficult to detect. These cells are the true homologues of the so-called vitreous cells of the diplostichous eye, e. g. of the Spider. (2) The eye is an ommateum, composed of ommatidia, the constituent cells of which extend from the corneal hypodermis to the fibrous basement membrane. (3) Each ommatidium is made up normally of four retinophoral cells surrounded by several circles of retinulae. (4) The four retinophorae are equal in all respects to one another; their outer ends are expanded into a calyx, which lodges four retinidia ( = vitreous bodies, supra), one to each cell; and these retinidia are placed axially instead of terminally, as in the Lamellibranch Arca and some Arthropoda. They vary much in consistency.
The middle portions of the retinophorae are contracted, and fused into a slender hyaline 'style' in which the axial nerve is contained; their basal portions may or may not expand into a 'pedicel.' This pedicel contains transverse striated plates, which appear to act like the argentea and tapetum combined in the eye of Pecten. The base of the pedicel contracts and the four retinophoral cells again become separate. There appears, however, to be a closer connection between two of them. (5) The pigmented retinulae bear no rods; the cells of the outermost circle in the region of the calyx are lighter, and contracted basally into bacilli; the cells of the circle surrounding the style or pedicel are darker, and sometimes, if not always, seven in number. They are continued outwards as a hyaline sheath to the calyx. Between the bases of the bacilli and the style or pedicel there may be a number of irregular cells containing highly refractile granules or crystals, soluble in weak hydrate of potash solution, to which they impart a red tinge. Similar crystals may be present in the outermost retinulae. The style or pedicel of the retinophorae coincides with the rhabdome so-called in the text.
Patten objects strongly to the terms 'vitrellae,' 'retinulate,' 'auto-' and 'exo-chromic' He believes that the corneal hypodermis is always present, and that the eye of Peripatus represents the primitive Arthropodan eye. For the definition of his terms, or of the terms as he uses them, see note, p. 452, ante. Points in the structure of the eyes upon which he lays stress are noted under the different classes of Arthropoda. The compound polymeniscous eye occurs also in the Lamellibranch Arca.
Respiration may be cutaneous, i.e. effected by the general surface of the body, in the lower Crustacea and Arachnida; or branchiate, i.e. effected by processes developed from the surface of the body, or in connection with the limbs, as in most Crustacea and some Arachnida; or tracheate, and carrying air to all the tissues, as in Insecta, Myriapoda, most Arachnida, and the Protracheata. The tracheae are developed as internal growths from the hypodermis, lined by a chitinoid cuticle, which is generally crenulated spirally in the tracheal tubes. The simplest form is seen in Protracheata as a depression of the integument, from which arises a bundle of tracheal tubes. The structure is much the same in Myriapoda Diplo-poda. But as a rule the tracheal tubes branch and anastomose. The external apertures are known as stigmata. In certain Arachnida (e.g. Scorpio) the depression is large, and its walls disposed in leaf-like folds, tracheal tubes being absent. Such structures form the so-called lungs or lung-books. The tracheal system may become closed externally in aquatic forms, and then external processes of the integument, variously shaped, may be developed, in which tracheal branches ramify, as in the tracheal gills of some Insecta. Air enters and is expelled from the open tracheal system by contractions of the body walls and by diffusion.