ARTHROPODA. 493 



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 Arthropodan 1 . Auditory 

 organs exist in the shape of special hairs in various regions of the body- 

 surface (Araneidae y 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 



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: (i) 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 Area 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 con- 

 nection 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 Area. 



