DEVELOPiMENT. 95 



mission. Upon this theory, the Pteropods and the Gastcro- 

 pods have proceeded from a common progenitor, and have 



■ B 



Fig. 24. — A, Young oi EolLs, a water-breathing Gasteropod, showing the provi- 

 sional buccal lobes. B, Adult Ptcropod (Z./w<it/;/rt Antarctica). After Wood- 

 ward. 



therefore inherited certain common characters. Since the 

 period, however, when they branched off from the common 

 stem, the Gasteropods have undergone much modification, 

 whereas the Pteropods have retained very much the char- 

 acters of the original stock. The adult Gasteropod comes, 

 therefore, to difter very much from the adult Pteropod ; but 

 the young Gasteropod, being as yet unspecialised, still pre- 

 sents characters derived from the primitive stock in an un- 

 modified form. 



Retrograde DEVELorMENT. — 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 tlie 

 adult is to all appearance a degraded form as compared 

 with the larva. This phenomenon is knowrt as ''retro- 

 grade " or " recurrent " development, and it is seen in its 

 most marked form in animals which lead a free life when 

 young, but are parasitic in their habits when fully grown, 

 though it is not exclusively confined to these. A striking 

 example of retrograde development is aflbrded by the smgu- 

 lar crustaceans known as Epizoa. In these the larval form 

 is free-swimming, provided with locomotive limbs, and fur- 

 nished with well-developed organs of \ ision, being in most 



