8 Invertebrate* 



from the mouth end to the opposite pole. They may 

 be grouped radially in one plane around this, as in the 

 star-fish (fig. 3), or disposed, as in higher forms, on 

 each side of a longitudinal vertical plane in the line 

 of the axis. When such bilateral symmetry exists the 

 surface undermost in progression (ventral) differs from 

 the opposite or dorsal surface, and the organs, instead 

 of lying in one plane, may be arranged in groups dis- 

 posed in successive segments along the axis, as in the 

 worm (fig. 34) or centipede (fig. 62). 



Different animals live in conditions requiring vary- 

 ing degrees of the action of the several organs, so the 

 development of the organs varies proportionally. In 

 fact animals are so perfectly fitted to their surround- 

 ings that could we know all the conditions under which 

 a given animal existed, we could form a good concep- 

 tion of its structure. 



The Embryo. We cannot understand the true 

 nature of animal structures without knowing the stages 

 of their formation from their first appearance in the 

 gg until they attain their adult condition. The im- 

 mature germ in the egg is called the embryo, and this 

 is not a simple miniature of the full-grown animal, but 

 first appears as a small mass of protoplasm, a single 

 cell, which undergoes division, thereby increasing into 

 a mulberry-like cluster of cells, which develops still 

 farther into a bilaminar sac. In process of growth the 

 primitive cavity of this sac becomes modified into the 

 characteristic inner cavity of each kind of animal ; 

 the outer layer of the sac becomes the outer layer or 

 cuticle of the animal's body, while the inner becomes 

 the lining of the digestive canal. An intermediate 

 layer developing between these two gives origin to the 



