64 COELENTERATA [CH. 



main kinds of sense organs are frequently found, which may be 

 roughly called eyes and ears; never, however, both kinds in one 

 Medusa. The " eyes " are little coloured patches of skin, some of 

 the cells of which end in clear rods, while others secrete a coloured 

 substance or pigment. Both pigment and rods are necessary if 

 there is to be a vision, though we do not understand why. The ears 

 are little pits in the base of the velum ; they may be open or 

 their edges may come together, so that the ectoderm lining them is 

 entirely shut off from the outer skin. In either case, some of the 

 cells forming the walls of the pits secrete particles of lime, others 

 close to them develop delicate sense hairs. The result is that 

 vibrations in the water, if they come with a certain frequency, will 

 affect the heavy particles, and their vibrations in turn will affect 

 the sense hairs. There is another kind of information, however, 

 which organs like these give their possessor, and this is probably still 

 more important to the floating Medusa, namely, information as to 

 the position of the animal with regard to the vertical. In other 

 words, the Medusa learns from them whether it is moving upwards 

 or downwards or sideways : for when the animal shifts its position, 

 the heavy particles in the ear- sacs are shifted comfortably and affect 

 different sense cells. 



It is a very interesting fact that " Eyed " Medusae or Antho- 

 medusae arise from hydroid stocks in which the perisarc is confined 

 to the base and in which the first or mother person is taller than the 

 daughters which sprout from her. Such forms are called Gymno- 

 blastea. " Eared " Medusae or Leptomedusae arise from stocks in 

 which the mouth and tentacles are covered by a cup of perisarc called 

 the " hydro thcca" and in which the stem is built up by a daughter 

 sprouting from the mother hydroid's neck and a grand-daughter 

 from the daughter's neck and such forms are called Calyptoblastea. 

 Graptolites or " Pen-stones " from the Ordovician slates of Cumber- 

 land are extinct Calyptoblastea. 



Through these different sense organs stimuli are continually 

 pouring in from the external world. If the stimuli only affected 

 the contractile cells nearest them irregular movements would result. 

 The function of the nerve ring, as of all nervous systems, is to co- 

 ordinate the stimuli, that is to collect and rearrange and rapidly 

 distribute them to the whole animal so that a definite reaction of 

 the whole contractile tissue results, not a series of local reactions 

 interfering with one another. 



The Medusa is very voracious and rapidly increases in size. It 



