Degeneration can be produced by other causes than parasitism. It is evident that if for any other reason an animal should adopt an inactive fixed life it would degenerate. The barnacles (see fig. 37) are excellent examples of degeneration through quiescence. They are crustaceans related most nearly to the crabs and shrimps. The young barnacle just from the egg is a six-legged, free-swimming larva (nauplius) with a single eye, greatly like a young prawn or crab. It develops during its independent life two compound eyes and two large antennæ. But soon it attaches itself to some stone or shell, or pile or ship's bottom, giving up its power of locomotion, and its further development is a degeneration. It loses its compound eyes and antennæ, and acquires a protecting shell. Its swimming feet become modified into grasping organs, and it loses most of its outward resemblance to the typical members of its class. The Tunicata or ascidians compose a whole group of animals which are fixed in their adult condition and have thus become degenerate. They have been likened to a "mere rooted bag with a double neck." In their young stage they are free-swimming, active, tadpole-like or fish-like larvæ, possessing organs much like those of the adult simplest fish or fish-like animals. Their larval structure reveals, however, the relationships of the ascidians to the vertebrates, a relationship which is not at all apparent in the degenerate adults. Certain insects live sedentary or fixed lives. All the members of one large family, the Coccidæ, or scale-insects (figs. 62 and 63), have females which as adults are wingless and in some cases have no legs, eyes, or antennæ, while the males are all winged and have legs and the special sense organs. The males lead a free active life, but the females have nearly or quite given up the power of locomotion, attaching themselves by means of their sucking beak to some plant, where they obtain a sufficient food-supply (plant-sap) and lay their eggs. In both males and females the larvæ are little active crawling six-legged creatures with legs, eyes, and antennæ.