420 



ELEMENTARY ZOOLOGY 



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 independ- 

 ent life two compound eyes and two large antennae. 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 de- 

 velopment is a degeneration. It 

 loses its compound eyes and an- 

 tennae, and acquires a protecting 

 shell. Its swimming feet become 

 modified into grasping organs, 

 and it loses most of its outward 

 resemblance to the typical mem- 

 bers 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 larvae, possessing 

 (From 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 rela- 

 tionship 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 Coccidae, or scale- 

 insects (figs. 62 and 63), have females which as adults are 

 wingless and in some cases have no legs, eyes, or antennae, 

 while the males are all winged and have legs and the 



sitic ichneumon fly. 

 specimen. ) 



