240 



CHAPTER XVI. 



SUB-KINGDOM CCELENTE RAT A— continued. 



CHARACTERS, STRUCTURE, AND DISTRIBUTION OF THE ACTINOZOA. 



The division of the Actinozoa comprises the Sea-anemones, the 

 Corals, and various allied forms, and is defined as including 

 Ccele?iterate animals in which the mouth opens into an oesophageal 

 tube, which in turn opens beloiu into the general cavity of the body 

 (" coelenteric space"). The oesophagus is separated from the body-wall 

 by an interveni?ig "perivisceral space" which is divided into a series 

 of compartments by radiating vertical membranous partitio7is or 

 " mesenteries" to the faces of which the reproductive organs are 

 attached. 



The Actinozoa differ, therefore, fundamentally from the Hydrozoa 

 in this, that whereas in the latter the space (" ccelenteric " space) 

 included within the body-walls is simple and undivided, and there 

 is no proper alimentary tube, in the former there is a distinct 

 oesophagus (fig. 120, g), and the general cavity of the body is di- 

 vided into radial compartments by vertical membranous plates 

 (" mesenteries "). 



The tissues of the Actinozoa consist of an external " ectoderm " 

 and an internal "endoderm," between which is developed an in- 

 termediate layer or " mesoderm " (the " mesogloea). The ectoderm 

 covers the entire outer surface of the organism, and is prolonged 

 inwards at the mouth to form the lining of the oesophageal tube, 

 while the endoderm lines all the internal cavities of the body. 

 Both the ectoderm and endoderm are primitively cellular, but both 

 are liable to undergo more or less differentiation, muscular fibres, 

 nerve-cells, thread-cells, &c, being developed in process of growth. 

 The mesoderm is essentially composed of connective tissue, and 

 forms an intermediate layer (the " Stiitz-lamelle " of the Germans) 

 which gives stability to the soft body of the animal. 



The Actinozoa may be simple, as in the Sea-anemones, the organ- 



