274 VENOMS 
This venom resists ebullition for fifteen minutes. 
V. Henri and Mdlle. Kayalof made experiments in immunisa- 
tion. Rabbits that receive every third day increasing doses of 
gemmiform pedicellariæ of Spherechinus granularis tolerate well, 
after four injections, the toxin of 40 pedicellariæ, a lethal dose. 
The serum of these rabbits is not protective for either rabbit, 
crab, or fishes. 
Frog serum (1 c.c.) injected into the body cavity of a crab, 
protects this animal against the pulp of pedicellariæ injected 
immediately afterwards. 
The pedicellariæ easily become detached from sea-urchins. 
They remain fixed to objects which come into contact with them, 
and the urchin abandons them like poisoned arrows. 
On touching a point on the surface of the body of an urchin, 
the spines are seen to incline towards the spot touched, and the 
pedicellariæ stretch themselves out and lean with their valves open 
towards the seat of the stimulus. In Spherechinus granularis the 
heads of the gemmiform pedicellariæ are covered with sticky mucus 
forming a tiny drop, visible under the lens. A specimen of this 
species possesses more than 450 pedicellariæ. 
C.—ARTHROPODS. 
(a) Araneida (Spiders). 
Almost all Arachnids possess poison-glands, which are con- 
nected, in some cases with the buccal apparatus, in others with 
a special inoculatory organ situated at the posterior extremity of 
the body. The spiders and scorpions belong to this group, and 
their venom is particularly active. 
On each side of the mouth of spiders is found an appendage 
ending in a fang (chelicera), at the extremity of which opens the 
excretory duct of a more or less developed poison-gland. The 
venom produced by these glands is instantly fatal to all small 
