B.—Echinoderms.
The Echinoidea (Sea-urchins) are provided with soft prehensile organs, the pedicellariæ, of which four kinds are distinguished: gemmiform, tridactyle, trifoliate, and ophiocephalous.
These pedicellariæ contain a special venom, which causes the paralysis and death of animals into which it is injected. Uexkull, who was the first to mention it, considered that the gemmiform pedicellariæ alone are toxic.
From this point of view various species of sea-urchins, Strongylocentrotus lividus, Arbacia æquituberculata, Sphærechinus granularis and Spatangus purpureus, have recently been studied by V. Henri and Mdlle. Kayalof.[110]
The pedicellariæ were removed and pounded up in sea-water, and the pulp was injected into crabs, holothurians, star-fish, cuttle-fish, frogs, lizards, and rabbits; in the case of cuttle-fish and rabbits the injection was made intravenously; in that of the other animals into the body-cavity.
For crabs the lethal dose was from 20 to 30 gemmiform pedicellariæ of Strongylocentrotus lividus.
The holothurians, star-fish, and frogs proved immune.
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.