THE EVOLUTION OF ORGANIC FORM. 85 



There are certain requirements of the animal organism to which 

 every adaptation must conform. Underlying the stomachic type is 

 the more primary fact that the natural form of colloid matter is the 

 globe. Like all fluid or semi-fluid matter it tends to curve about a 

 general center of attraction, in distinction to the angular extension of 

 the crystal. This tendency shows itself in all parts of all animal forms, 

 and also in these forms as wholes, the globe being departed from only 

 through functional necessity, or from the superposition of a series of 

 organs, each with a globular tendency, yielding, through mutual pres- 

 sure, a more or less ovoid result. 



In the Protozoa we have the globular form, diversified by temporary, 

 improvised limbs, or by permanent organs. In the Metazoa variation 

 from the globe takes place in axial directions the fixed animals hav- 

 ing usually but two axes of departure from the sphere ; the moving 

 animals having ordinarily three axes a longitudinal, a vertical, and a 

 transverse. The general result is the production of the round, flattened 

 form of the two-axed, and the oval form of the three-axed animals ; 

 the further departure being in the production of limbs appendages 

 devoted to motion, or to assault and defense. 



If now we take the Gastrula, the simple stomach-sac, for the primi- 

 tive form of the many-celled animal, and the earliest phase of deriva- 

 tion from the Protozoa, it is easy to perceive that this hollow animal 

 globe may vary in three different modes. 



First, it may retain its sac-like form and stomach-opening, devel- 

 oping tentacles about the mouth, and radiated body divisions ; thus 

 passing from the single axis of the Gastrula to the double axis of the 

 polyp. 



Secondly, it may flatten, until it resembles a sack with the open top 

 pressed down upon the bottom, and the sides bulging outward into a 

 circle. If, now, radiated arms extend outward from this rounded side, 

 we have the starfish type of organism. 



Thirdly still preserving its affiliation to the globe it may lengthen 

 instead of flattening. From this mode of development would come 

 the longitudinal type of animals, the vermes, or worm-forms. 



A still more primitive departure from the original Gastrula form is 

 found in the sponge, in which the body- wall is pierced by minute aper- 

 tures, through which food-bearing currents are drawn into the general 

 internal stomach, and forced out again through the mouth. The low 

 organization of the sponge results from the fact that it does not even 

 require the mouth-arms of the polyp as an aid in food-getting. Its 

 only motive apparatus is the cilia, or vibrating hair, of the Infusoria. 



Of the three forms which thus seem to be the first natural varia- 

 tions of the Gastrula the globular, the flattened, and the lengthened 

 the first two naturally rest on one extremity of the longitudinal or 

 stomach axis ; the other, or mouth extremity, being directed upward. 

 Thus only these two extremities are exposed to diverse conditions, the 



