32 SUMMARY OF CURBENT RESEARCHES RELATING TO 



normal specimens appeared to prefer pale red against the 8 in 

 dark blue ; of blind individuals, 536 were found in the first, and 406 

 in the latter colour ; with colours of about equal intensity, 474 were 

 found in the red, and 176 in the blue. 



The proportion of individuals preferring a good light devoid of 

 ultra-violet rays was as 2 to 1 of those found in darkish ultra-violet 

 light ; as between green and blue, the proportion was 3 to 1 of the 

 respective colours for unblinded, and about 1| to 1 for blinded in- 

 dividuals. Thus blinded animals are shown to be sensitive to both 

 quantitative and qualitative differences in light, 



Graber considers the above facts to be in accordance with the 

 theory of evolution of special optical organs (eyes) from generalized 

 ones (skin) ; as the reactions of these hypothetical dermal organs 

 resemble those of the former, and their inferior activity is quite 

 natural. This agreement favours the interpretation of the phenomena 

 as due to an inferior degree of vision, and not to the results of thermal 

 or chemical influences acting on the animals experimented on. 



B. INVERTEBRATA. 



Nerve-centres of Invertebrata. * — W. Vignal has examined the 

 nervous system of various groups of the higher invertebrates and 

 comes to the following, among other, conclusions : — 



In the Crustacea the cells of the ganglia are nearly all unipolar, 

 and almost always consist of a viscous granular substance, in which 

 the nucleus is slightly and the nucleoli highly refractive. Bipolar 

 and multipolar cells are also present. The nerve-fibres forming the 

 connectives, the commissures, and the nerves have a proper wall, on 

 the surface or in the interior of which there are oval nuclei ; the 

 inclosed substance is viscid and slightly granular, and contains a 

 central bundle of fibrils, or the fibrils are isolated. The central 

 nerve-chain and the nerves are invested in two sheaths, one of which 

 is structureless, and appears to be of a cuticular nature, while the 

 other is formed of imbricated lamellge, which, in the macrourous 

 Crustacea, forms a partition in the connectives. The nerve-cells on 

 the ventral face of a ganglion send off prolongations into its centre ; 

 this centre is formed of nerve-fibres, and of prolongations from the 

 cells ; the two are closely united and form a plexus whence the 

 nerves are given off. The gastro-intestinal nerves are composed of 

 fine fibres which have the same structure as those of the ventral chain. 

 They form two plexuses, along which nerve-cells are to be observed. 



In the Mollusca bipolar or multipolar cells are very rarely found 

 among the cells of the ganglia, and this is especially the case in the 

 Gasteropoda. The nerve-cells are formed of a ganglionic globe on 

 the surface, and in the interior there are fine fibrils ; among these are 

 fine fatty granulations, which are sometimes variously coloured. The 

 ganglionic globe, which has no investing membrane, contains a large 

 nucleus and one or more nucleoli. The nerves and connectives are 

 formed by fibres of very various sizes, which are separated from one 



* Arch. Zool. Expe'r. et Gen., i. (1883) pp. 267-408 (4 pis.). 



