There are many species of the salpæ, but they bear a closer likeness to each other than do most of these simple tribes of being. In form they all resemble diminutive purses, composed of highly transparent jelly, with wide mouths like the ordinary clasp—and strengthened by a net-work of ribbons interwoven with the general texture of the purse. These are designed to supply the place of muscles. The salpæ move through the water by contracting the net-work, so as to render the cavity smaller and expel the water from it with some force; then, relaxing the fibres, they allow their natural elasticity to expand them to their original form; thus drawing in a fresh supply of fluid with which to renew the effort. In this manner they are driven onward, always retreating from the principal orifice of the sac. But I will not detain you with a detailed description of their singular organization. It is enough for our present purpose to state that near the bottom of the purse, within the thickness of its walls, there is a golden spot, as if a solitary coin was there deposited. This spot alone enables us to see the animal distinctly when floating in the water. When young, these little creatures adhere together in strings or cords arranged like the leaflets of a pinnated leaf, in consecutive pairs, to the number of twenty or more. At that period, the most common species in the South Atlantic rarely exceed one half an inch in length, and the yellow spot hardly equals in size an ordinary grain of sand; yet, in certain regions of the ocean these salpæ swarm in such inconceivable multitudes that the sea assumes the appearance of a sandy shoal for miles in length and breadth. To the depth of many fathoms their delicate bodies are closely huddled together, until the constant repetition of the diminutive colored spots renders the water perfectly opaque, and so increases its consistence that the lighter ripple of the surface breaks upon the edge of the animated bank, while the heavier billows roll on smoothly, with the regular and more majestic motion of the ground swell. In passing through such tracts the speed of the vessel is sometimes sensibly checked by the increased resistance of the medium in which she moves; and when a bucket full of brine is lifted from the sea, it may contain a larger portion of living matter than of the fluid in which it floats. There can be no reasonable doubt that most of those false shoals which disfigure the older charts—their existence proved upon authorities of known veracity and denied by others no less credible—have really been laid down by navigators who have met with beds of salpæ, and were ignorant of their true nature.