202 SUMMARY OF CURRENT RESEARCHES RELATING TO 



7. Brachiopoda. 



Recent Brachiopoda.* — The late Dr. T. Davidson's monograph of 

 recent Brachiopoda is now completed. The two groups Arthropomata 

 and Lyopomata each contain three families ; the former, which has the 

 greater number of species, is divided into the Terehratulidse, in wliich a 

 number of subfamilies are recognized, the Thecidiidse, and the Ebyn- 

 chonellidse ; and there are in all seventeen genera or subgenera. The 

 Lyopomata embrace the CraniidaB, Discinidse, and Lingulidse, the only 

 subgenera being Discinisca and Glottidia. Some additional notes are 

 added by Miss A. Crane, who has had the editorial charge of the 



work. 



Arthropoda. 



Vision of Arthropods.f — Prof. F. Plateau gives a short summary of 

 the results of his long continued experiments on the phenomena of vision 

 in Arthropods. Those that have no eye, such as certain Myriopods, 

 distinguish light from darkness. These dermatoptic perceptions very 

 probably exist in most Arthropods whether or no they have visual 

 organs, and they explain most of the special facts presented by in- 

 dividuals who have been artificially blinded. In Arthropods with simple 

 eyes only, vision is, as a rule, very bad. Some, like Myriopods, Spinning 

 Spiders, and Phalangida, do not seem to perceive the form of a body at 

 any distance at all ; others, like Hunting Spiders, Scorpions, and larvae, 

 seem to see the contours of objects more or less confusedly ; but the 

 distance seen is always small. A large number of Arthropods perceive 

 the displacements of mobile bodies; all aid their insufficient visual 

 organs by a skilful use of the organs of touch. Notwithstanding the 

 absence of a power of really distinct vision, in the sense understood of 

 Vertebrates, there are three chief factors which cause Arthropods with 

 simple eyes to move about with sufficient adroitness to provide food, and 

 to sometimes present such a bearing as to lead a superficial observer to 

 believe that they are endowed with good sight. When an Arthropod 

 has both compound and simple eyes the latter are of hardly any use. 



An Insect with compound eyes has no sharp perception of form ; from 

 the functional point of view facetted eyes are inferior to the eyes of 

 Vertebrates. Though they have no complete perception of form some 

 perceive movements which are not too rapid, as do the Lepidoptera, 

 Hymenoptera, Diptera, and Odonata ; at distances which vary between 

 58 cm. and 2 metres these animals see displacements of objects of a 

 certain size much better than they see the objects themselves. What, in 

 a general way, happens with a flying insect is this — the animal moving 

 in air with a very lively perception of light and shadow is able to avoid 

 masses, such as trunks of trees, rocks, or walls, and passes them at a 

 suitable distance. If by chance he should be in the midst of underwood 

 or any other group of plants, he profits by the solutions of continuity 

 through which light is filtered, or which offer him the largest surface. 

 If the wind agitates the leaves the openings oscillate, but, thanks to his 

 power of perceiving movement, he sees them all the better ; he describes, 

 in flying, undulations so as to follow the direction of the displacements, 

 and to get out without injuring himself. When his mode of feeding 



* Trans. Linn. Soc. Lond., iv. pp. 1-74, pis. i.-xiii. (1886) ; pp. 75-182, pis. 

 xiv.-xxv. (1887) ; pp. 183-248, pis. xxvi.-xxs. (1888). 



t Bull. Acad. R. Sci. Belg., Iviii. (1888) pp. 395-457 (1 pi.). 



