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 tracheae 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 book. 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 
3 Treviranus Arachnid. 1 - . 1. 1./. I. r. f. 10. Comp. A r . 
Diet. d'Hist. Nat. xxx. 419. Latreille calls these gills Pncumo- 
b ranches. 
