tissue is defined as a group of cells having 

 similar structure and function and ar- 

 ranged in a compact, organized array. 

 Thus, we talk of connective tissue or bone 

 tissue or epidermal tissue. The revolu- 

 tionary transformation of multicellular 

 organisms from quasi-amorphous con- 

 glomerates of cells into structures with 

 well-defined tissues was a significant 

 step, for it enabled these tissues subse- 

 quently to combine into organs and or- 

 gan systems and thus permitted the 

 levels of complexity that higher animals 

 have since attained. 



The most primitive group of organ- 

 isms still extant that displays a definite 

 tissue organization is the Phylum Coelen- 

 terata (or Cnidaria as it is now called). 

 This is a group of fresh-water and marine 

 animals that includes Hydra (a favorite 

 laboratory animal in elementary biology) 

 and its relations, the sea anemones that 

 of O Primitive abound on rocky coasts, the corals, the 



jellyfish, and the awesome Portuguese 

 Man of War, whose toxin has paralyzed 

 The many unwary swimmers and led to death 



by drowning. A brief resume of the gen- 

 CoelenterateS eral properties of these organisms illus- 



trates their primitive condition: 



1. The body is composed of only 

 two tissues, the outer epidermis and the 

 inner gastrodermis. These arise early in 

 the development of the coelenterate em- 

 bryo. In contrast, the tissues of all higher 

 animals stem originally from three em- 

 bryonic cell layers. Within the two coe- 

 lenterate tissue layers are a number of cell 

 types, some of which appear in Fig. 18. 

 37 



The 

 Development 



Animal 



