THE COMMON PLAN OF ANIMALS. 285 



gastrula, as the early stages of development. The like 

 is true of all the worms, sea-urchins, starfishes, jellyfishes, 

 polypes, and sponges ; and it is only in the minutest and 

 simplest forms of animal life that the germ, or repre- 

 sentative of the ovum becomes metamorphosed into the 

 adult form without the preliminary process of division. 



In the majority even of these Protozoa, the typical 

 structure of the nucleated cell is retained, and the whole 

 animal is the equivalent of a histological unit of one of 

 the higher organisms. An Amc?ba is strictly comparable, 

 morphologically, to one of the corpuscles of the blood of 

 the crayfish. 



Thus, to exactly the same extent as it is legitimate 

 to represent all the crayfishes as modifications of the 

 common astacine plan, it is legitimate to represent all 

 the multicellular animals as modifications of the gastrula, 

 and the gastrula itself as a peculiarly disposed aggregate 

 of cells ; while the Protozoa are such cells either isolated, 

 or otherwise aggregated. 



It is easy to demonstrate that all plants .are either 

 cell aggregates, or simple cells ; and as it is impossible 

 to draw any precise line of demarcation, either physio- 

 logical or morphological, between the simplest plants, 

 and the simplest of the Protozoa, it follows that all forms 

 of life are morphologically related to one another ; and 

 that in whatever sense we say that the English and the 

 Californian crayfish are allied, in the same sense, though 

 not to the same degree, must we admit that all living things 



