56 SUMMARY OF CURRENT RESEARCHES RELATING TO 



An account is then given of the development of the eye in Pecten. Inter 

 alia, Mr. Patten notes the occurrence of transitijry pigmented cups, 

 prohably homologous with the invaginated eyes of Area. At the base of 

 the ophthalmic fold, between it and the velum, a few large cells form small 

 oval thickenings, the rudiments of the future eye. By continued prolifera- 

 tion of the cells on the outer side of the optic thickening an oval knob-like 

 papilla is formed. The proliferation continues to form a hypodermic core. 

 At first this is not sharply defined, and several of the deeper cells separate 

 from the rest and mingle with the numerous connective-tissue cells. They 

 are the ganglionic cells, which later provide the eye with nerve-fibres. The 

 optic vesicle and several of its parts are then accounted for in detail. An 

 interesting comparison between the origin of the sense-papillae and the 

 origin of the optic nerves is suggested, and a future elaboration promised. 

 The sense-hair paj)illaB appear as thickenings of the hypodermis similar to 

 those of the eyes. An inward proliferation of cells forms an ectodermic 

 core, which becomes transformed into the longitudinal nerve with which 

 every tentacle is provided. 



Patten's observations are then extended to Ostrea, Mactra, Pinna, 

 Avicula, Cardium, and Haliotis. The chapter on Arthropods is reported 

 on separately.* Some of his suggestive conclusions on Mollusca are sum- 

 marized below. 



General. — The term ommatidium is redefined by Patten to designate 

 those constructive elements of all eyes consisting of single (?) or compound 

 colourless cells — the retinophorae, surrounded by one or more circles of 

 pigmented ones, or retinulae. A typical molluscan ommatidium consists 

 of a double, colourless, sometimes gland-like cell, the retinophora, con- 

 taining two nuclei and an axial nerve-fibre. The external surface of the 

 latter cijntains nerve-fibres, breaking up externally into fibrillse continuous 

 w'ith those from the axial nerve, and forming a network or retinidium, 

 usually supported by a cuticular secretion or rod. The pigmented cells 

 round the retinophora may also exhibit retia terminalia in a cuticular 

 secretion. 



The visual organs of Molluscs are referable to four types ; the diflFuse, 

 the invaginate, the fiicetted, and the pseudo-lenticulate, representing the 

 three modifications of ligLt-sensitive surfaces, i.e. a retineum, an om- 

 mateum, and a retina. (An ingenious comparison is made between the 

 chromatophores of Ce2)halopoda and the isolated ommatidia.) The relation 

 of these four types is shown, the invaginate have originated from the 

 pigmented pits, and the facetted from the invaginate form. The frequent 

 presence of an unnecessary number of eyes is explained by the hypothesis 

 that eyes originated from organs having other functions, viz. from organs 

 for the absorption of energy from the sunlight, or heliophags. 



The high epithelial cells of Molluscs end at the base in several root- 

 fibres attached to the basal membrane, and are frequently pigmented ex- 

 ternally and capped by a double cuticula. In nearly all nerve-fibres extend 

 along the wall of the cells towards their outer ends. A rete terminale 

 is present throughout the whole hypodermis, and especially well developed 

 near the eyes. In some cases large unbranching nerve-fibres pass directly 

 to the cuticle, and seem to end in sense-hairs. These large fibres expand 

 below the basal membrane into nucleated bipolar vesicles. 'ihis basaJ 

 expansion wanders into the underlying tissue, while the outer end becomes 

 educed to fibre, but still terminates into one or more sense-hairs. The 

 outer end gives rise to minute cross fibrillae which unite with others, the 

 tuft of hairs disappears, and the sense-cell has become a bipolar ganglionic 



♦ Injia. p. 82. 



