32 MANUAL OF ZOOLOGY. 



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constitute its development. The egg which is laid bv a butter- 

 fly undergoes a series of changes, which eventuate in its giving 

 birth to a caterpillar, these preliminary changes constitut- 

 ing its transformation. The caterpillar grows rapidly, and 

 after several changes of skin becomes quiescent, when it is 

 known as a ' chrysalis.' It remains for a longer or shorter 

 time in this quiescent and apparently dead condition, during 

 which period developmental changes are going on rapidly in 

 its interior. Finally, the chrysalis ruptures, and there escapes 

 from it the perfect winged insect. To these changes the term. 

 metamorphosis is rightly applied. These changes, however, do 

 not differ in kind from the changes undergone by a mammal ; 

 the difference being that in the case of a mammal the ovum is 

 retained within the body of the parent, where it undergoes 

 the necessary developmental changes, so that at birth it has 

 little to do but grow, in order to be converted into the adult 

 animal. 



From these considerations we arrive at the second law laid 

 down by Quatrefages : ' Thos^ creatures whose ova owing to 

 an insufficient supply of nutritious contents and an incapacity 

 on the part of the mother to provide for their complete de- 

 velopment within her own substance are rapidly hatched, 

 give birth to imperfect offspring, which, in proceeding to their 

 definitive characters, undergo several alterations in structure 

 and form, known as metamorphoses.' 



Retrograde Development. Ordinarily speaking, the course 

 of development is an ascending one, and the adult is more 

 highly organised than the young ; but there are cases in which 

 there is an apparent reversal of this law, and the adult is to 

 all appearance a degraded form when compared with the 

 embryo. This phenomenon is known as 'retrograde,' or 

 'recurrent' development, and well marked instances are found 

 amongst the Cirripedia and Lernea9, both of which belong to 

 the Crustacea. 



Thus, in the Cirripedes (acorn- shells, &c.) and in the 

 parasitic Lernese the embryo is free-swimming and provided 

 with organs of vision and sensation, being in most respects 

 similar to the permanent condition of certain other Crustacea, 

 such as the Cypris (Ostracoda). The adult, however, in both 

 cases, is degraded into a more -or less completely sedentary 

 animal, more or less entirely deprived of organs of sense, and 

 leading an almost vegetative life. As a compensation, repro- 

 ductive organs are developed in the adult, and it is in this 

 respect superior to the locomotive, but sexless, larva. 



