214 SUMMARY OF CURRENT RESEARCHES RELATING TO 



codomic cavities of the somites. (7) Eye-spots arise from thickening of 

 hypodermis, and formation of pigment-lined vcsielo. The front wall 

 thins into lens, the cells of most internal wall and sides become retinal ; 

 tlio pigment-cells of Grenadier are probably mesodermic ; a connection 

 with the ganglion-cells of nervous system is early established. 



The most striking feature is the reduction of the ventral, and the 

 increase of the dorsal part of the young animal. The relations are the 

 same as those in carboniferous Euphoberia. Each double segment 

 represents two complete segments, the dorsal plates of which have fused 

 into one. 



S. Arachnida. 



Vision in Arachnids.* — Prof. F. Plateau, in continuation of his pre- 

 vious memoirs, gives an account of the observations which have been made 

 by himself and by others on the power of vision exhibited by Arachnids. 

 After describing his separate experiments with about a dozen species 

 of spiders, he sums up the general results as follows : — (1) The 

 Araneidaj in general perceive at some distance the displacements of large 

 objects; (2) the hunting spiders (Attidae, Lycosidae) are probably the 

 only forms that see the movements of small bodies ; (3) they perceive 

 these movements at a distance which varies in different species from 

 2-12 centimetres ; (4) the distance at which the prey is seen distinctly 

 enough to induce an attempt to capture it, is only 1-2 cm. ; (5) even at 

 this slight distance the vision is not exact, for the hunting spiders make 

 numerous errors ; (6) the non-hunting web-making spiders have very 

 poor vision, they only perceive the presence and the direction of their 

 prey by the vibrations of the filaments, and will seek to capture little 

 bodies quite other than insects if the vibrations produced on the web be 

 somewhat similar. 



The author then discusses the vision of scorpions. His experiments 

 with Buthus europseus, taken along with Ray Lankester's observations on 

 other species, show that the vision is very poor ; that the distance of 

 distinct sight is not more than 1 cm. for the median eyes, and 2^ cm. for 

 the laterals ; that the animals do not really hunt, but rather wait on luck ; 

 that feeling is, both in locomotion and in dealing with their prey, vastly 

 more important than sight. 



Lastly, Prof. Plateau discusses the Phalangidas. His experiments 

 show that they stand at about the same sensory level as the web-making 

 spiders. The vision is very poor, distinct sight hardly at all developed 

 for any distance. The Phalangidte make up for this, however, by the 

 exquisite development of their appendages, and especially of their 

 pedipalps. 



Respiration of Arachnida.t — Prof. F. Plateau has made some 

 experiments by the graphic method which demonstrate the absence of 

 perceptible respiratory movements in these animals. There is some doubt 

 as to whether there are any transverse muscular fibrils in the pulmonary 

 plates of Arachnids ; none such arc figured by Prof. Ray Laukester, 

 and Mr. Locy says expressly that he has failed to demonstrate the 

 muscular differentiation described by M. MacLeod. Prof. Plateau, 

 having performed his part as experimenter, looks to the histologist to 

 resolve the disputed point in minute anatomy. 



* Bull. Acad. R. Sci. Bulg., xiv. (1887) pp. 545-95 (1 pi.). 

 t Arch, de Biol., vii. (1S87) pp. 333-48. 



