D.—Molluscs.
Certain Gastropodous Molluscs, chiefly Murex brandaris and M. trunculus, possess purple glands from which it is possible to extract a very active venom (Raphaël Dubois)[136] by crushing them up with sand and alcohol. The alcoholic liquid, filtered and evaporated in a water-bath, yields a brown oily fluid. The subcutaneous injection of a few drops of this into a frog is sufficient to produce very decided toxic effects. Sluggishness and slowness of movement are seen to supervene fairly quickly; reflex actions are still exhibited, but the animal is no longer able to jump.
If the dose be not too strong, this condition of paresis lasts for several hours, and then disappears. In most cases, however, the paresis is succeeded by complete paralysis, and the animal appears as though suffering from curare. Yet the fact is that the venom is neither curare-like nor cardiac; the heart, muscles, motor endplates, and motor and sensory nerves are spared; the nervous centres alone are attacked, especially the encephalon. The animal dies without convulsions.
Sea and fresh-water fishes (golden carp) are very sensitive to this venom; warm-blooded animals are refractory. It is therefore probable that, in the species of Murex, the purple gland is a poison-gland serving for defence, or for the capture of the prey upon which these molluscs feed.
Among the Cephalopods, the Octopods (Octopus vulgaris, common octopus, Eledone moschata, musky octopus, of the Mediterranean) possess two pairs of salivary glands, a small anterior pair, and a posterior pair of considerable size.
The Decapods (cuttle-fishes [Sepia], &c.), have only posterior salivary glands, of smaller dimensions in proportion to the size of the body.
On being crushed and macerated in water, the anterior glands yield a limpid and slightly acid juice; the posterior glands produce a viscid, ropy fluid, filterable with difficulty and neutral. The latter has an immediate paralysing effect upon Crustacea. It contains a substance of a diastasic nature, precipitable by alcohol, and destructible by heating for an hour at 58° C.
Owing to the poisonous properties of this juice, Octopods succeed in overpowering large prey, such as lobsters and crabs. Once they are seized by the tentacles of the octopus, or cuttle-fish, a bite inoculates these animals with venom that immediately destroys their power of movement, and the Cephalopod is able to continue its meal in perfect security, without having to fear the pincers of its prey.
An experimental study of this venom has been made by A. Briot,[137] who found that crabs are very sensitive to it, while rats, frogs, rabbits, and fish do not appear to experience any inconvenience.
CHAPTER XVII.
VENOMS IN THE ANIMAL SERIES (continued).
2.—VENOMOUS FISHES.
The means of defence in fishes are extremely varied. Some species (torpedoes or electric rays, electric eels) destroy their enemies by electric discharges; others are provided with true poison-glands and inoculatory organs, usually represented by opercular spines or by the fin-rays. The species of the genus Muræna, however, possess a poison-apparatus connected with the buccal teeth, as in the case of snakes.
It has been clearly established by Bottard[138] that at least three very distinct types of venomous fishes exist, according as the venom-apparatus is:—
(1) Entirely closed (Synanceia type); (2) half closed (Thalassophryne type); (3) in more or less direct communication with the exterior (Trachinus and Scorpæna type).
The greater part of the following statements has been borrowed from the excellent work of the author referred to, from the writings of A. Corre,[139] the fellowship thesis of Henry Coutière,[140] and the magnificent atlas published at St. Petersburg in 1886 by P. Savtschenko, of the Russian Imperial Navy.
Except in the case of the species of Muræna, the venom of fishes is generally found in one or more special glands, situate at the base of the dorsal or caudal fins, or beneath the opercular spines. When the animal defends itself it inflicts wounds with these rays, and ejects from its poison-glands a toxic or irritant liquid, which enters the sores.
The flesh of these fishes is not usually poisonous, whereas a fairly large number of other species, which do not inflict wounds, cause intoxicating effects when eaten. These latter do not come within the scope of this work; but the reader who may desire to obtain information with regard to them will find them well described in J. Pellegrin’s memoir,[141] in that by Dupont, and especially in the papers of A. Corre.
Venomous fishes almost all belong to sedentary species, as in the case of the genera Trachinus, Cottus, Scorpæna, and Synanceia. This fact suggested to Dissard and Noë[142] a very hazardous theory in order to explain the existence of a poison-apparatus in these animals. The venomous fishes being sedentary, say these authors, have no need of a poison-apparatus; their prey offers itself to them without effort on their part, and, on the other hand, they escape destruction by their enemies. If, therefore, they possess a poison-apparatus it is because the conditions under which they live entail the lowest value for the co-efficient of respiration, diminish the quantity of the ambient radiations and the oxygenation of the medium, and lead to diminished hæmatosis. For these reasons the activity of anaerobic life becomes greater, and the formation of venoms takes place.
This theory, derived from the conceptions of A. Gautier with regard to the formation of toxic leucomaines, appears scarcely tenable, for it is evident that the weever, for example, erects its first dorsal spine as soon as it is seized, and that Scorpæna and Synanceia likewise protrude their venomous spines when conscious of danger. The poison-apparatus of these fishes is therefore of an eminently defensive character.
According to Bottard, the spawning season increases the activity of the poison-glands and at the same time the toxicity of the secreted product. Several species, such as those of the genus Cottus and the perch, possess no apparent secreting cells except at this period. Certain toxicophorous or poisonous fishes, such as the species of Tetrodon, are particularly noxious at the time when their genital glands are at their maximum activity.