610 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



the periphery. The lens is kept in its place by a very delicate 

 fibrillated tissue, perhaps representing the vitreous humour, inter- 

 calated between the lens and the retina. The latter, like the simple 

 eyes of the Diptera, is composed of rods and large fusiform cells, 

 each of which is continuous with a fibre of the optic nerve. 



Marine Caddis-fly.* — Mr. E. M'Lachlan describes and figures the 

 larva of a caddis-fly (Pliilanisus plebejus Walker) which inhabits 

 rock pools between high and low water marks in New Zealand, and 

 forms its case of coralline sea-weed. No truly marine form has 

 hitherto been recorded, though at least one species lives in the 

 brackish water of the shores of the Baltic, and several are found in 

 salt marshes or pools occasionally invaded by the sea. 



y. Araclmida. 



Respiratory Organs of Arachnids, j — J. Macleod, in a pre- 

 liminary account of his observations on these organs, commences by 

 stating that his earlier researches had led him to regard the lungs of 

 Arachnids as a special form of trachea, or, in other words, as modified 

 tracheae, and in support of this it is pointed out that,- while the 

 tetrapneumonous Araneids have two, and the dipneumonous forms one 

 pair of lungs, the latter have a pair of tracheal stigmata, comparable 

 to the same organs in insects. 



The lungs may be regarded as consisting of a cavity or chamber, 

 the hinder portion of which opens to the exterior by means of a trans- 

 verse slit, the lips of which are pro- 

 vided with a thickened chitinous pad ; 

 the chitinous lining of the cavity is 

 continuous, at the stigma, with the 

 cuticle of the external integument. 

 Into the cavity there extend the lung 

 lamellse ; each of these is composed of 

 two chitinous layers, the spaces be- 

 tween which are occupied as blood'- 

 passages ; for all the passages there is 

 a common vestibule (Fig. 109), and the 

 slits of all are similar to one another, 

 with one exception ; the last, instead of 

 being cylindrical, is more or less tri- 

 angular, its chitinous cuticle is thick 

 and carries a large number of spines 

 well developed, and so arranged as to 

 form a kind of second tunic. 



With regard to the tracheae in the 

 Araneida, we see that they may be 

 simple or branched, and their stigmata confluent or separated. In 

 an Argyroneta, in which they are well developed, there are two 

 large cylindrical primary trunks, which open to the exterior by two 



* Journ. Linn. Soc. (Zool.) xvi. (1882) pp. 417-22 (5 figs.), 

 t Bull. Acad. R. Sci. Belg., iii. (1882) pp. 779-92. 



Fig. 109. 



