94 



PRINCIPLES OF ANIMAL BIOLOGY 



Organization in Simple Metazoa. — In the simpler metazoa organiza- 

 tion has not gone as far as outlined above. In the sponges, for example, 

 there is no differentiated nervous system (Fig. 32), but any of the ex- 

 posed cells are capable of receiving stimuli and conducting impulses in 

 addition to their other functions. In the sponge there are but few con- 

 tractile cells. These occur singly or in very small groups, and their 

 function is to close up the pores through which the water passes. The 

 germ cells are scattered and do not form tissues. The cells which line the 

 radiating canals and which are therefore widespread in the animal carry 

 on digestion. There is no circulating medium which carries food in solu- 



FiG. 60. — Hydra, diagrammatic representation of a sagittal section, hi, b2, buds in 

 different stages of growth; ec, ectoderm; em, endoderm;/, foot; gvc, gastrovascular cavity 

 or coelenteron; m, mouth; ov, ovary; t, tentacle; ts, testis. 



tion, but digested materials diffuse through the digestive cells into neigh- 

 boring tissues. There is no tissue specialized for the respiratory function 

 since all cells which come into contact with water carry on respiratory 

 functions in addition to other functions, nor are there any special cells for 

 the elimination of waste materials. From these few statements it is 

 obvious that the cells of the sponge are but little removed from protozoan 

 cells in their degree of specialization. 



Hydra and its allies differ from the sponge in the greater development 

 of cells capable of contraction and in the grouping of reproductive cells 

 into spermaries or ovaries. The cells of the outermost layer (Fig. 60) 

 serve several functions, such as contraction, conduction of stimuU, support 



