36 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Arthropod Vision.* — Dr. Em. Radl considers that those who have 

 discussed this subject have not realised the importance of a study of 

 the nerve-centres of the eye, as well as of the dioptric apparatus itself. 

 His belief that the key to the problem of Arthropod vision is to be 

 sought in the central rather than in the peripheral organs, has led him 

 to make an exhaustive examination of the optic tract in Squilla mantis, 

 as well as of the eye itself. He briefly describes the external appear- 

 ance of the eye, and gives a very comprehensive account of the occur- 

 ence of the phenomenon of " double eyes " — well known to occur in 

 Squilla — in other Arthropods, both aquatic and terrestrial. He then 

 somewhat briefly describes the eye itself, and gives a full account of 

 the structure of the eye-ganglia. Each ommatidium gives oif seven 

 nerve-fibrils which unite in a bundle, and as these bundles cross the 

 space between the basal membrane of the eye and the first ganglion 

 (ganglion retinas), those from neighbouring ommatidia unite to form 

 larger bundles. In the first ganglion these bundles break up into their 

 constituent fibrils. This first ganglion, like the eye itself, is made up 

 of two halves, which are connected by a thick bundle of vertical nerve- 

 fibres. The ganglia have a complicated structure, an especially im- 

 portant element being the granular layer, which contains darkly staining 

 bodies, the " nerve-nodes " (Nervenlcnoten), corresponding in number 

 to the ommatidia. These consist of neuroglia-fibrils, of a homogeneous 

 substance with the staining reactions of neuroglia-fibrils, and especially 

 of a group of nerve-fibrils, to whose presence the node is due. It would 

 seem that each node contains fibrils from several ommatidia, but the 

 fibrils apparently do not end at the node, but pass on through this first 

 ganglion to the other ganglia behind. As the fibrils leave the first 

 ganglion to pass to the second, they cross so that those at the right side 

 become left, and vice versa. The importance of this crossing the author 

 believes lies in the varying lengths of the fibres, for he thinks that this 

 variation in length has a direct physiological significance. This sig- 

 nificance he explains as follows. Suppose the eye to be stimulated in 

 such a way that a certain set of retinulse receive an equal impulse. 

 These impulses travel down the fibrils to, e.g. the second ganglion, but 

 as the lengths of the paths they have to travel vary, they will not 

 arrive simultaneously, but one after the other according to the relative 

 lengths of the fibrils. In general terms it may be said that every 

 adequate stimulus will produce a periodic cycle of changes in the re- 

 ceptive organ. Even if we suppose a stimulus to affect one ommatidium 

 only, there would be set up a successive series of changes in the nerve- 

 centres, for each ommatidium, as noticed above, has seven nerve-fibrils, 

 and each of these has (apparently) a course of different length from 

 those of its neighbours. This theory of Arthropod vision was reached 

 by the author as the result of a process of induction, but he believes 

 that it receives confirmation from the theories of other authors, e.g. 

 Exner, who have based their conclusions on theoretical grounds. 



Variations in Crest of Daphnia hyalina. f — Miss M. M. Enteman 

 discusses the variability in the cephalic crest of this species. Every 

 kind of crest displayed in the genus may be observed in the species, 



* Zeitschr. wise. Zool., lxvii. (1900) pp. 551-98 (1 pi.). 

 t Amor. Nat., xxxiv. (1900) pp. 879-90 (6 figs.). 



