164 AN INTRODUCTION TO ZOOLOGY 



patch of pigment ed epithelium, in which are a number of sense cells. 

 This area is sensitive to light, and so forms a very rudimentary sort 

 of eye, the ocellus. 



The mouth leads straight into the enteron, which continues 

 up the manubrium, and at its base, within the thickness of the 

 umbrella, enlarges to form a rounded rectangular cavity, the stomach. 

 From the corners of the stomach four delicate tubes, the radial 

 canals, pass out to the periphery, there to open into another tube, 

 the circular canal, which runs round the edge of the umbrella. 

 About half-way to the circumference each radial canal has a small 

 downwardly directed sac, around which the germ cells are congre- 

 gated, forming an outbulging in the sub-umbrella surface, the gonad. 

 In this way we have what is, compared with a polyp, a complex 

 gastro-vascular system, which is throughout lined by entoderm. 

 The radial canals are not isolated, but connected together and with 

 the circular canal by a very delicate double layer of cells, forming 

 a thin sheet known as the entodermal lamella. All the remaining 

 part of the thickness of the umbrella is made up by the jelly-like 

 mesoglea, which fills up all the spaces between the external covering 

 ectoderm and the internal entoderm. It corresponds with the 

 mesoglea of the polyp or Hydra, and is secreted by the other layers, 

 but is very much thicker. 



Like the polyp, the medusa is radially symmetrical about a main 

 axis, constituted by a line passing through the centre of the ex- 

 umbrella and the mouth. Four secondary axes are marked out by 

 lines drawn through the radial canals, which also pass through the 

 arms of the cruciform mouth. These are distinguished as the 

 perradii, and the canals in consequence are sometimes termed the 

 perradial canals. The axes formed by bisecting the angles between 

 these perradii are termed the interradii, and the eight axes half-way 

 between these and the perradii are "the adradii. The tentacles at 

 the end of all these different radii, sixteen in number, are the first 

 to be formed, and the medusa may be set free at this stage, but not 

 before and often not until other tentacles have appeared. Each 

 adradial tentacle has not only an ocellus, but in addition another 

 sense organ in the form of a minute spherical sac. This is lined with 

 ectoderm, and the cells at its lowest part possess sensory hairs, while 

 small calcareous particles are present inside. These bodies are the 

 lithocysts or statocysts. From a somewhat superficial resemblance 

 to the sensory parts of the membranous labyrinth in the ear of 

 higher animals they were formerly termed otocysts, but this is not 

 a good term. There is little doubt that their function is the per- 

 ception of the position of the animal in space, particularly its 

 orientation with regard to the action of gravity, and hence they are 



