198 THE COCKROACH : 



CEPHALOPODA, which are all marine, have no ciliated band, 

 and the post-embryonic changes do not amount to metamor- 

 phosis. There is usually a much larger yolk-sac than in other 

 Mollusca. 



CRUSTACEA usually pass through well-marked phases. Pcneus 

 presents five stages of growth (including the adult), the earlier 

 being common to many lower Crustacea. The Crab passes 

 through three, beginning with the third of Peneus ; the Lobster 

 through two ; while the freshwater Crayfish, when hatched, is 

 already in the fifth and last. 



*/ 



FISHES seldom undergo any post-embryonic change amount- 

 ing to metamorphosis. Ampliioxus (if Amphioxm be indeed a 

 fish) is the only well-marked case. 



AMPHIBIA develop without conspicuous metamorphosis, except 

 in the case of the Frogs and Toads (Anura), which begin life 

 as aquatic, tailed, gill- bearing, and footless tadpoles. 



REPTILES, BIRDS, and MAMMALS do not undergo transformation. 



This survey, hasty as it necessarily is, shows that habitat is a 

 material circumstance. Larval stages are apt to be suppressed 

 in fluviatile and terrestrial forms. Further, it would seem that 

 zoological rank is not without influence. Metamorphosis is 

 absent in Cephalopoda, the highest class of Mollusca, and in all 

 but the lowest Vertebrates, while it is almost universal in 

 Coelenterates, Echinoclerms, and Lamellibranchs. 



It has often been remarked that the quantity of food-yolk 

 indicates the course of development. If a large store of food 

 has been laid up for the young animal, it can continue its 

 growth without any effort of its own, and it leaves the egg well 

 equipped for the battle of life. Where there is little or no 

 yolk, the embryo is turned out in an ill-furnished condition to 

 seek its own food. This early liberation implies metamorphosis, 

 for the small and feeble larva must make use of temporary 

 organs. Some very simple locomotive appendages are almost 

 universally needed, to enable it to get away from the place of its 

 birth, which is usually stocked with as much life as it can support. 



Some animals, therefore, are like well-to-do people, who can 

 provide their children with food, clothes, schooling, and pocket- 

 money. Their fortunate offspring grow at ease, and are not 



