INTERNAL ANATOMY OF INSECTS. 61 



may be denominated false gills, or branchiform appen- 

 dages, I shall now call your attention to what may be 

 denominated true ones, which are peculiar to the Arach- 

 nida Class : but what is remarkable, the animals that 

 breathe by them are very rarely inhabitants of the water, 

 so that their functions cannot be perfectly analogous to 

 those of fishes. 



In the Scorpion, on each side of the four first ventral 

 segments a spiracle may be discovered, which has no 

 lip as in other insects, but is merely a circular orifice. 

 These orifices do not lead to trachea or vesicles, but to 

 true gills, which are situated below a muscular web which 

 clothes the internal surface of the crust. Each gill con- 

 sists of many semicircular very thin plates, of a dead 

 milky white, which are connected together at the dorsal 

 end like the leaves of a b ( ook. There appear to be more 

 than twenty of these leaves, which when strongly mag- 

 nified look transparent and destitute of any vessels. 

 Each gill is fastened at the back to the spiracle a . In the 

 spiders also, gills are discoverable, but differently cir- 

 cumstanced. On the under side of the abdomen, near 

 the base, is a transverse depression, on each side of which 

 is a longitudinal opening leading to a cavity, which is 

 covered from above by a cartilaginous plate. In this 

 cavity is situated a true gill, which is white, triangular, 

 and covered with a fine skin ; the leaves of this gill are 

 far more numerous and much finer and softer than those 

 of the gills of the scorpion. On account of their softness 

 they have often the appearance of a slimy skin; but their 



a Treviranus Arachnid. 7~. t. I./. 1. r. f. 10. Comp. N. 

 Diet. d'H'wt. Nat. xxx. 419. Latreille calls these gills Pneumo- 

 branches. 



