IV MEDUSA 311 



and muscle-fibres of the sub-umbrella and velum, some of 

 which differ from the similar structures in the hydranth in 

 exhibiting a delicate transverse striation. 



There is still another important matter in the structure of 

 the medusa which has not been referred to. At the junction 

 of the velum with the edge of the umbrella there lies, imme- 

 diately beneath the ectoderm, a layer of peculiar, branched 

 cells, containing large nuclei and produced into long fibre- 

 like processes. These nerve-cells (p. 167) are so disposed 

 as to form a double ring round the margin of the bell, one 

 ring (Fig. 77, I), ww) being immediately above, the other 

 («»') immediately below the insertion of the velum. An 

 irregular network of similar cells and fibres occurs on the 

 inner or concave face of the bell, between the ectoderm and 

 the layer of muscle-fibres. The whole constitutes the 

 nervous system of the medusa;. the double nerve-ring is the 

 central, the network ^& peripheral nervous system (p. 155). 



Some of the processes of the nerve-cells are connected 

 with ordinary ectoderm-cells, which thus as it were connect 

 the nervous system with the external world : others, in some 

 instances at least, are probably directly connected with 

 muscle-fibres. 



We thus see that while the manubrium of a medusa has 

 the same simple structure as a hydranth, or what comes to 

 the same thing, as a Hydra, the umbrella has undergone a 

 very remarkable differentiation of its tissues. Its ordinary 

 ectoderm cells, instead of being large and eminently con- 

 tractile, form little more than a thin cellular skin or epith- 

 elium (p. 1 09) over the gelatinous mesogtea : they have 

 largely given up the function of contractility to the muscle- 

 processes or fibres, and have taken on the functions of a 

 protective and sensitive layer. 



Similarly the function of automatism, possessed by the 



