The Tunicata, or sea squirts, are animals which have become simple through degeneration, due to the adoption of a sedentary life, the withdrawal from the crowd of animals and from the struggle which it necessitates.  The young tunicate is a free-swimming, active, tadpolelike, or fishlike creature, which possesses organs very like those of the adult of the simplest fishes or fishlike forms.  That is, the sea squirt begins life as a primitively simple vertebrate.  It possesses in its larval stage a notochord, the delicate structure which precedes the formation of a backbone, extending along the upper part of the body below the spinal cord.  The other organs of the young tunicate are all of vertebral type.  But the young sea squirt passes a period of active and free life as a little fish, after which it settles down and attaches itself to a shell or wooden pier by means of suckers, and remains for the rest of its life fixed.  Instead of going on and developing into a fishlike creature, it loses its notochord, its special sense organs, and other organs; it loses its complexity and high organization, and becomes a “mere rooted bag with a double neck,” a thoroughly degenerate animal.

A barnacle is another example of degeneration through quiescence.  The barnacles are crustaceans related most nearly to the crabs and shrimps.  The young barnacle just from the egg is a six-legged, free-swimming nauplius, very like a young prawn or crab, with a single eye.  In its next larval stage it has six pairs of swimming feet, two compound eyes, and two antennae or feelers, and still lives an independent free-swimming life.  When it makes its final change to the adult condition, it attaches itself to some stone, or shell, or pile, or ship’s bottom, loses its compound eyes and feelers, develops a protecting shell, and gives up all power of locomotion.  Its swimming feet become changed into grasping organs, and it loses most of its outward resemblance to the other members of its class.

Certain insects live sedentary or fixed lives.  All the members of the family of scale insects (Coccidae), in one sex at least, show degeneration that has been caused by quiescence.  One of these coccids, called the red orange scale, is very abundant in Florida and California and in other fruit-growing regions.  The male is a beautiful, tiny, two-winged midge, but the female is a wingless, footless, little sack, without eyes or other organs of special sense, which lies motionless under a flat, thin, circular, reddish scale composed of wax and two or three cast skins of the insect itself.  The insect has a long, slender, flexible, sucking beak, which is thrust into the leaf or stem or fruit of the orange on which the “scale bug” lives, and through which the insect sucks the orange sap, which is its only food.  It lays eggs under its body, and thus also under the protecting wax scale, and dies.  From the eggs hatch active little larval “scale bugs,” with eyes and feelers, and six legs.  They crawl from under