Page:Popular Science Monthly Volume 16.djvu/661

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Hydra or of a Medusa. In one Agalma we find, as in the Hydractinia, nourishers supplied with one long tentacle, of which a single touch produces a severe burning sensation, a sort of fish-line, which in large species is capable of capturing fishes. Besides the nourishers,

PSM V16 D661 Medusa and campanularida.png
Fig. 6.—Hidden-eyed Medusa. Fig. 7.—Gonophore of one of the Campanularida.

are found individuals without a mouth, which are only reproducers, in the neighborhood of which are sexual individuals resembling Medusæ in form. All these individuals are fixed upon a common axis, which floats like a serpent in the water, where it is sustained by an air-vessel forming its superior extremity. Two series of sterile Medusæ appear underneath this bell, a gang of oarsmen (physales), to which the colony abandons the care of locomotion.

These various parts are in all respects too much like the Hydras and Medusæ for us to refuse them the character of individuals; the Agalma and other Siphoniferæ are true societies or colonies. But here most of the individuals can not separate themselves without danger of death; and, in certain cases, they all coördinate their movements that the colony may perform certain acts. For example, in the Portuguese men-of-war (Fig. 8) the physales frequently change their course, and then all the individuals of the colony concur in the operation. They have, then, a will which controls them—a will that can find the grounds of its decisions only in a sort of social consciousness, elevating the colony to the rank of a psychological unit. Composed of individuals each of which is equivalent to those Hydras or Medusæ that live free and isolated and sufficient for themselves, every Siphonophore must still be considered in its turn as a single animal