1 8 CRUSTACEA. 



between the structure and symmetry of the human form 

 and that of the creature immediately before the pupil. 



The agreements in external symmetry, that it has 

 eyes, feelers, or sense organs, and a mouth forming an 

 anterior end remotely comparable with our own head; 

 that it can be divided by an imaginary plane passing 

 through the centre into halves, equal bilaterally like the 

 human form ; that the appendages as limbs for support 

 in walking or swimming are in such bilateral animals, 

 as in our own case, necessarily distributed on the sides ; 

 that such bodies which seek their food must necessarily 

 have the mouth and organs of sense at one end, and 

 the organs for locomotion either along the sides or at 

 the posterior end, are all perfectly legitimate and nat- 

 ural comparisons arising from the common character- 

 istics of all animals which move freely, seeking food, 

 self-preservation, and reproduction. The different 

 means and structures which have been produced to 

 meet the common requirements of life upon the surface 

 or in the waters of the earth, are also strongly illus- 

 trated by such comparisons, when made with discre- 

 tion, especially between these free-moving types and 

 such sedentary forms as the Sponges, the Hydra, and 

 the Corals, which have radiate symmetry like plants, 

 mouths or openings above, and no heads. 



The body should be studied first, and afterwards the 

 appendages.* The body is seen to be long and cylin- 



* Scholars should, of course, he allowed a certain latitude, 

 and this by no means narrow or pedantic, in making their ob- 

 servations, though at the same time they should be led gradually 

 to consider a certain natural order as convenient and essential 

 to the clear presentation of their ideas. 



