UNITY OF DESIGN. 631 



and multiple organs of digestion; and the 

 central filaments of the nervous system in both 

 being studded with numerous pairs of equidis- 

 tant ganglia. In the worm all these features 

 remain as permanent characters of the order ; in 

 the insect they are subsequently modified and 

 altered during its progressive metamorphoses. 

 The embryo of a crab resembles in appearance 

 the permanent forms of the Myriapoda, and of 

 the lower animals of its own class, but acquires, 

 in the progress of its growth, new parts ; while 

 those already evolved become more and more 

 concentrated, passing, in their progress, through 

 all the forms of transition which characterise 

 the intermediate tribes of Crustacea; till the 

 animal attains its last state, and then exhibits 

 the most developed condition of that particular 

 type.* 



However different the conformations of the 

 Fish, the Reptile, the Bird, and the Warm 

 blooded Quadruped, may be at the period of their 

 maturity, they are scarcely distinguishable from 

 one another in their embryonic state ; and their 

 early developement proceeds for some time in 



* This curious analogy is particularly observable in the suc- 

 cessive forms assumed by the nervous system, which exhibits a 

 gradual passage from that of the Talitrus, to its ultimate great- 

 est concentration in the Maia. (See Figures 439 and 441, p. 

 543 and 545.) Milne Edwards has lately traced a similar pro- 

 gression of developement in the organs of locomotion of the 

 •Crustacea. (Annales des Sciences Naturelles; xxx, 354.) 



