In the case of rabbits weighing 1½ kilogrammes, 40 pedicellariæ of _Sphærechinus granularis_, pounded up in 1 c.c. of water, produce death by asphyxia and general paralysis in from two to three minutes. The heart continues to beat after respiration has ceased. For lizards and fishes the toxic dose is the same as for the crab. The cuttle-fish is paralysed and killed in two hours by 50 pedicellariæ. This venom resists ebullition for fifteen minutes. V. Henri and Mdlle. Kayalof made experiments in immunisation. Rabbits that receive every third day increasing doses of gemmiform pedicellariæ of _Sphærechinus 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 _Sphærechinus 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 connected, 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 animals upon which spiders feed. In man and large mammals their bite produces sensations of pain accompanied by swelling and muscular contractions as though caused by localised tetanus. The venom of certain species of spiders sometimes causes very serious and even fatal results. _Latrodectus malmignattus_ (the _malmignatte_ of the South of France and Italy), and especially _Latrodectus mactans_, of Chile (fig. 96), are greatly dreaded.[111] The area of distribution of the latter includes the whole of Tropical and Sub-tropical America. It is said that it frequently causes the death of milch cows, and that in man its bite produces tetanic effects, which last for several days, but are in most cases amenable to treatment. [Illustration: FIG. 96.--_Lactrodectus mactans_ (_formidabilis olim_).