188 iSlARVELS OF POND-LIFE. 



the cells of which appear to be in every respect 

 similar to one another." 



The same writer adds that comparatively few of 

 the lower animals ''came forth from the egg under 

 their adult forms. The change is sometimes obvi- 

 ou.sly gradual, as in the progressive advance of the 

 tadpole into the condition of the frog; but it is 

 sometimes apparently sudden, as when the chrysalis 

 skin is thrown off, and the perfect insect comes 

 forth. In the latter case, however, the change is 

 really just as gradual as in the former, since the 

 organs characteristic of the perfect insect are under- 

 going development during the whole of the chrysalis 

 period, to be displayed and brought into use at its 

 termination." 



Every subordinate form of animated being bears 

 some resemblance to the embryonic stage of a higher 

 animal, and we moreover observe that organs and 

 functions, which in the higher animals are insepar- 

 ably bound together as portions of one whole, are 

 divided in the lower ranks. Thus sponges scarcely 

 do more than absorb and assimilate; entrails and 

 scarcely anything else constitute the structure of 

 the elegant Physalia^ or "Portuguese man-of-war;* 

 and they may be regarded as organs for those purposes. 

 The male rotifer is not a complete animal, as we 



* "Beautiful as the Physalia is, it is entirely a system of entrails 

 floating witli the waves." — Dr. Harveifs ''Sca-Stde Book." 



