256 COMPARATIVE ANATOMY 



walls of the sac are formed of two distinct cell layers cemented 

 together, as it were, by an intermediate structureless substance, 

 the mesoglcea. An imaginary line drawn through the mouth 

 to the centre of the surface of attachment marks out the 

 longitudinal axis of the body. The tentacles are disposed 

 radially with regard to this axis. There is a single internal 

 cavity communicating with the exterior only by the mouth, and 

 this cavity is at once the receptacle of food, the organ of diges- 

 tion, and the means whereby the products of digestion are 

 distributed. This structural plan, exhibited in its simplest 

 form in Hydra, is diagnostic of the Coelenterata, but is. variously 

 modified and disguised in the different classes into which that 

 phylum is divided. Particular attention should be given to 

 the fact that there are only two primary cell layers, ectoderm 

 and endoderm, in the coelenterate body, and that these corre- 

 spond with the outermost and innermost germ-layers the 

 epiblast and hypoblast of the frog's embryo. The third middle 

 layer, or mesoblast, is entirely absent in Ccelenterates (for the 

 mesoglcea is not a cellular layer), and hence these animals are 

 sometimes distinguished as diploblastic or two-layered animals. 

 The name Coelenterata calls attention to the important char- 

 acteristic of a single internal cavity which, from its fulfilling 

 the functions of digestive and circulatory systems, is called the 

 gastro-vascular cavity. 



Careful microscopical examination of the ectoderm and 

 endoderm shows that these two layers are differentiated so as 

 to perform various functions which in the higher three-layered 

 animals are performed by the mesoblast. Both ectoderm and 

 endoderm, being layers of cells spread over external or internal 

 surfaces, come under the definition of epithelia (p. 80). 

 The ectoderm, covering the outside of the body, is essentially 

 the protective and sensory layer; the endoderm, lining the 

 internal cavity, is essentially the digestive and secretory layer. 

 The two layers become continuous with one another at the 

 lips of the mouth. 



Partly because of their delicacy, partly because of the 

 intimate union of their component cells, the tissues of Hydra 

 are very difficult to study. Two methods are available. The 

 animal may be killed and hardened by plunging it suddenly 

 in a mixture of acetic acid and corrosive sublimate, or, prefer- 

 ably, in the mixture of chromic, osmic, and acetic acids, known 



