REPRODUCTION AND DEVELOPMENT 



569 



bipinnaria settles to the bottom and changes 

 into a minute, radially symmetrical starfish. 



Metamorphosis in insects 



Perhaps the most striking of all types of 

 metamorphosis occurs among the insects 

 (Figs. HO and 141). In this class the char- 

 acter of the metamorphosis, whether com- 

 plete or incomplete, is an important means 

 of classification. No metamorphosis takes 

 place in certain orders; in other orders, the 

 larvae or nymphs are recognizably similar to 

 the adults and are active throughout their 

 immature life; but in most of the insects, 

 metamorphosis is complete, and the cater- 

 pillars, grubs, maggots, etc., become quies- 

 cent pupae. Within the pupa, many of the 

 larval tissues break down into a liquid form, 

 which is utilized by groups of larval cells 

 called imaginal disks, to build up both in- 

 ternal and external organs of the adult. The 

 adult eventually emerges from the pupal 

 covering. 



hibited by the frog (Fig. 238) and many 

 other amphibians. 



PARENTAL CARE 



While parental care of eggs and young 

 is widespread in the animal kingdom, in 

 some instances, it may be lacking entirely. 

 Certain aquatic forms such as starfishes and 

 sea urchins shed their eggs and sperms into 

 the water and allow currents to bring them 

 together. Under these conditions, enor- 

 mously large numbers of gametes must be 

 produced to insure a sufficient number of 

 fertilized eggs to prevent the race from dy- 

 ing out. Other aquatic species lay their eggs 

 after fertilization where conditions are fa- 

 vorable for their development; for example, 

 the eggs of fish and frogs are laid in the 

 water and those of certain turtles in the 

 warm sand. The newly hatched young must 

 shift for themselves, since their parents pay 

 no attention to them. 



Metamorphosis in chordates 



Several very conspicuous cases of meta- 

 morphosis among the chordates are worth 

 mentioning besides those already studied. 

 Among the tunicates (Fig. 206), the tad- 

 polelike larva with its notochord, neural 

 tube, digestive tract, heart, pharynx, and gill 

 slits, becomes attached and undergoes re- 

 trogressive metamorphosis, during which 

 most of the larval organs are lost or become 

 degenerate. Larval lampreys were given the 

 genus name Ammocoetes before their rela- 

 tion to the adult lamprey was known. They 

 resemble the amphioxus (Fig. 201) very 

 closely in structure, and seem to represent 

 a sort of intermediate stage between primi- 

 tive chordates and primitive vertebrates. 

 After about 8 years of free life, the ammo- 

 coete larva acquires the structure and habits 

 of the adult lamprey. Metamorphosis from 

 an aquatic larva with gills to an air-breathing 

 terrestrial or semiterrestrial condition is ex- 



Care of eggs 



Protection and food are provided by the 

 parents of certain animals, who then seem 

 to consider their duty done, and they give 

 their offspring no further consideration. Egg 

 cases are common protective devices, such 

 as the cocoons of the earthworm (Fig. 97), 

 cockroach, and other insects. The solitary 

 wasps have become famous, since they not 

 only hide their eggs in a burrow or in a 

 protective nest, but they deposit near them a 

 supply of partially paralyzed insect larvae 

 for the young to feed upon when they hatch. 



Eggs may be cared for by parents in var- 

 ious ways. The crayfish attaches them to her 

 swimmerets (headpiece, p. 197); the water 

 flea Cyclops carries them about with her in 

 two egg sacs; certain spiders carry them in 

 a silk cocoon, and the fresh-water clams may 

 hold their eggs and young in their gills 

 throughout the winter (Fig. 177). The 

 Surinam toad carries eggs in epidermal 



