EMBRYONIC AND LARVAL HISTORIES 59 



radically and variously than the adult structure, but 

 the falsity of the extension of Von Baer's law depends 

 upon the fact that the progressive change in an 

 animal's structure does not necessarily take place in 

 the adult stage alone, but may affect any or all of the 

 processes of acquiring that stage. Even on purely 

 theoretical grounds, we do not know if, quite apart 

 from any active adaptive change in an animal's de- 

 velopment, the mere alteration of the adult structure 

 may not involve changes in the early structure and 

 development. 



However this may be, it cannot be denied that 

 embryonic and larval histories often throw a quite 

 unique light upon the relationship of adult animals, 

 which could not be guessed in any other way, and this 

 implies that the embryonic development of all the 

 organs of an animal must be taken into account in 

 estimating its relationship just as much as, if not 

 more than, their adult structure. 



As an example we may take the peculiar group of 

 Cirripede Crustacea, the Rhizocephala, which live as 

 parasites upon other Crustacea, chiefly crabs and 

 hermit crabs 1 . The adult structure of these parasites 

 is so highly degenerate that no one could guess that 

 they were Crustacea at all; in fact the Neapolitan 

 naturalist, Cavolini, who first discovered them, did 



1 The commonest of these parasites is Sacculina, found on the 

 shore-crab (Carcinus) and on the spider-crab (Inachus). 



