ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 411 



is the apparent inversion of the layers, for the epidermis appears to be 

 formed from the inner layer of cells, and the hypoblast from cells tbat 

 are, morphologically speaking, part of tbe outside layer of the embryo ; 

 the cause may perhaps be found in the so-called amnion. 



Tbe species of Peripatus appear to fall into three groups : New- 

 Zealand species, Cape species, and South American species. Tbe only 

 anatomical difference between the two latter which is of importance is 

 tbe presence of a receptaculum ovoruni, or closed vesicle, between the 

 ovary and tbe receptaculum seminis. The only other instance of great 

 variation in development which the author remembers is that of Bateson's 

 Balanoglossus and the ordinary Tornaria-Balanoglossus ; but tbat difference 

 is explicable by the difference in habit, for the former is mud-living and 

 the latter is pelagic. But, with Peripatus, the habits and mode of life 

 seem to be much the same wherever they occur, so tbat the striking 

 differences in development cannot be explained by change of habits 

 modified by external conditions. 



S. Arachnida. 



Eyes in Scorpions.* — Mr. G. H. Parker finds that the retinae of the 

 median and lateral eyes of scorpions are hypodermal in origin. The 

 median eye is found to be triplosticbous, and to be formed by an invo- 

 lution of the hypodermis and an inversion of tbe middle layer ; the first 

 layer (lentigen) is modified hypodermis immediately external to tbe 

 pocket of involution ; in addition to secreting the lens, it serves the pur- 

 pose which gained for it its earlier name of vitreous. The lens differs 

 from ordinary cuticle in having no pore-canals, and, except the external 

 hyaline layer, it can be stained throughout. The lentigen can produce 

 cuticula independently of the general hypodermis. The second layer, or 

 retina, is inverted, and consists of nerve-end-cells and pigment-cells ; it 

 contains phaospheres. The walls of the nerve-end-cells are thickened 

 into prenuclear rhabdomeres, and a nerve is given off from their deep ends ; 

 five rhabdomeres unite to form one rhabdome. Each pigment-cell forms 

 two sacs, connected by a stiff fibre ; tbe nucleus is in the inner one. The 

 third or post-retinal layer is the " sclera matrix " of Graber, and it 

 becomes intimately fused with the retina. In the embryo tbe fibres of 

 the optic nerve emerge from the external ends of the inverted retinal 

 cells, but in the adult from the opposite ends. The basement membrane 

 is a cuticiila formed by the inner ends of the hypodermal cells ; the 

 preretinal membrane is the united basement membranes of tbe lentigen 

 and retina ; the sclera is the basement membrane of the post-retina. 



The lateral eyes are monostichous, and arise from a simple thickening 

 and depression of the hypodermis ; around tbe margin of the depression 

 is a ring of perineural cells which secretes the lens ; they differ from tbe 

 lentigen in not having a vitreous function, owing to subsequent recession 

 removing them from between tbe lens and retina. The lens has tbe same 

 structure as in the median eye, but tbe retina wants the phaospheres ; there 

 is no preretinal membrane. Mr. Parker thinks that tbe lateral eyes 

 may well be supposed to represent the ancestral type of the median eyes. 



So-called Auditory Hairs.t — Herr W. Wagner has investigated the 

 nature of the hairs which Dahl described as " auditory." He distin- 



* Bull. Mus. Comp. Zool. Camb., xiii. (1887) pp. 173-208 (4 pis.), 

 t Bull. Soc. Imp. Nat. Moscou, 1888, pp. 119-34. 



