Post- 

 Embryonic. 



52 THE CONSTITUTION OF THE GASTROPOD PKOTOCONCH, 



(Phylembryonic) stage and adolescence. This metamorpho- 

 sis will always involve changes leading to the loss of the lar- 

 val organs, and may or may not involve others leading to the 

 acquisition of adult structures. In the absence of larval 

 organs there may yet be a metamorphosis leading to the 

 acquisition of adult characters only, such is almost the case 

 in many Entomostraca. We are thus able to recognise, in 

 very many instances, five distinct stages in the life-history of 

 the individual, and in some cases a sixth:—- 



l.Pre-larval ... Up to and including Jackson's j 



Typembryo. r Embryonic. 



2. Larval ... ... Phyle77ibryo(Ja,ckson). j 



3. Metamorphosis... jYepionic.(liuckma.n & Bather's 



name). 

 4.Juvenescence ... Neanic(li. & B.). 

 5. Prime of Life ... EphebicyH. & B.). 

 e.Old Age ... Gerontic(B. & B.). 



In many cases, these stages are so merged, that, to define 

 them, is much like attempting to divide the thin from the 

 thick end of the wedge, the whole life-history taking the form 

 of a gradual evolution. In some cases, the whole metamor- 

 phosis is passed through and juvenescence entered upon 

 before the loss of the larval organs ; for instance, in the 

 Mammalia, where the last portion of intra-uterine life, two 

 to three months in the human species, is potentially juvenes- 

 cent, and may be made actually so by premature birth. 



Difficulties present themselves more especially in the iden- 

 tification of the early stages, and these diflficulties of applica- 

 tion will, I fear, make it well-nigh impossible to give always 

 the same value to Jackson's terms for the subdivision of the 

 embryonic stage. The Phylenihryo, however, should always 

 be identifiable, as the last definitely embryonic stage, and is 

 represented by many characteristic larval forms. 



The attempt has been made, in one or two instances, to 

 define the stage of development by reference to the time of 

 liberation from the maternal organism, or escape from the 



