82 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the endoderm. In discussing the musculature, Eeichenbach raises the pro- 

 blem of the origin of the body-cavity, and notes especially that the 

 mesoderm-cells in the abdomen are congregated in masses representing the 

 segments, and that these exhibit a lumen, apparently progressive from 

 before backwards. This certainly appears at a late stage, but seems com- 

 parable with the segmented body-cavity of related types. Finally the 

 author was able to find after prolonged search what seemed to be the first 

 rudiments of the reproductive organs, but was unable to determine from 

 what layer they originated. 



Eyes of Crustacea.* — Mr. W. Patten gives an account of the structure 

 of the eyes of several Crustacea, and especially of those of Penseus. 



(a) The cornea is divided into square facets, and consists of tvv^o layers. 

 Below the cornea is a thin, continuous layer — the corneal hyjpodermis — to 

 which the corneal cuticle owes its origin. (h) Beneath this is the much 

 thicker ommateal hypodermis, of numerous ommatidia each consisting of 19 

 or 20 very long cells, extending down to the basal membrane. They are 

 arranged in four circles, and the nuclei of each group are at the same 

 level, in specially enlarged or pigmented portions. (c) The innermost 

 group consists of four colourless cells — the relinopliorse, forming an inverted 

 pyramid. The outer ends of the retinophora3 are thickened and contain 

 the nuclei ; below the nuclei the cells are filled vnih. a mass of less con- 

 sistent, finely granular protoplasm ; then follow the conical, four-cornered 

 crystalline cones, which are nearly half as long as the ommatidia them- 

 selves. The narrowing apex of each of these square pyramids is reduced 

 to the slender, tube-like " style." The final exiDanded solid portion forms 

 the rhabdom or pedicel. Near the basal membrane, the latter diverges into 

 three legs composed of the attenuated, inner ends of the four retinophoraB, 

 two of which have united. Each leg of the stalk is divided at its inner end 

 into several fibres by which it is united to the basal membrane. The seg- 

 ments of the so-called rhaodoms of Grenacher are not secretions of the 

 retinulse (or pigmented cover-cells), but the inner ends of the retinophorae, 

 which terminate in the same root-like fibres as occur in nearly all hypo- 

 dermic cells. 



(d) After giving an account of the complicated structure of the pedicels, 

 Patten passes to the retinulse. Seven of these oddly-shaped, pigmented 

 cells surround the retinophorae. The outer parts of the retinulfe seem to 

 terminate with the knob-like swellings containing the nuclei, but they 

 are really continued onwards as extremely delicate membranes. These 

 form a sheath round the style, though this is not always evident. The 

 pigmentation and relations of the retinulse are described in detail, (e) The 

 pigmented collar of the retinophoraB is formed by a third circle of four cells 

 in two pairs. The cells are continued inward to the basal membrane as 

 slender colourless rods or bacilli, and outwards to the surface of the omma- 

 tidium, as four delicate fibres, producing four minute impressions at each 

 comer of a corneal facet. The cell-stalks or bacilli, which are fastened 

 by root-like fibres to the basement membrane, are elongated, hyaline 

 fibres, with node-like thickenings at intervals. It is striking that these 

 prominent and simple structures have hitherto remained unobserved. In 

 the spaces between the diminished inner ends of the ommatidia is a third 

 group of cells, ensheathing the inner ends of the retinulas. They contain 

 a mass of yellowish, fat-like crystals. 



(/) The hasal membrane is extremely complicated. It consists of con- 

 nective tissue fibres, sometimes fused to form hyaline masses, connected 



* MT. Zool. Stat. Ncapel, vi. (1880) pp. 542-75G (5 pis.). Cf. Mollusca, supra, p. 53. 



