132 ELEMENTS OF BIOLOGY 



terates are, in the most simple types, vase-shaped, the walls consist- 

 ing of the outer or ectoderm layer, and the inner, or endoderm 

 layer. The ectoderm is charged with the functions of support and 

 protection, of receiving and transmitting stimuli, and of performing 

 such contractions as are necessary for the capture of food or for 

 locomotion. Differences in the functions of cells are always asso- 

 ciated with differences in structure; hence the ectodermal cells are 

 not all identical but are of several types. As the ectodermal cells have 

 become difiFerentiated for special purposes they lack the power of 

 digesting food. Digestion for the entire animal is carried on by the 

 cells of the inner layer, or endoderm. In these simple types of multi- 

 cellular animals occur a number of resemblances to the Protozoa. 

 As was stated previously (p. 91) perhaps the most prominent of 

 these in the Ccelenterata is that the endodermal cells ingest solid 

 particles as do the Protozoa; they retain, so to speak, the property of 

 carrying on intracellular digestion. The cavity of the vase-shaped 

 body (Fig. 79) also receives enzymes from the endodermal cells; it 

 forms a primitive digestive cavity, for within it the digestive en- 

 zymes carry on extracellular digestion. The products of digestion 

 in the cavity pass by osmosis into and through the endodermal cells 

 and are assimilated by both endoderm and ectoderm. 



Triploblastic Animals. In the diploblastic animals the ecto- 

 derm contains both contractile cells and cells that are more sensitive 

 to stimuli than others. Thus it performs both muscular and nervous 

 functions. In the triploblastic, or three-layered, animals, the ecto- 

 derm is the layer of origin of all purely nervous elements, including 

 the brain of the higher animals, but the property of contractility as 

 a function of the ectoderm is largely lost, being assumed by cells 

 that are derivatives of an intermediate layer, the mesoderm (Fig. 

 78). The mesoderm is also the layer of origin of the connective tis- 

 sues, of blood and the circulatory system, of the excretory system, 

 and of the reproductive cells and associated organs. The endoderm 

 is universally the layer of origin of all parts of the digestive system 



