THE CGELENTERATA 165 



organs of balance or equilibration. In these structures and in the 

 ocelli we have a decided advance upon the conditions obtaining in 

 Hydra or even the polyps of Obelia, for here we have definite parts 

 specialised for a particular function, in other words sense organs. 

 In correlation with this sensory system we find that the nervous 

 system also is more highly organised, for in addition to the ordinary 

 scattered nerve cells in the mesoglea there are also developed two 

 much denser rings of these cells around the margin of the umbrella, 

 although even yet we have no definite aggregations to form 

 ganglia. By means of the marginal nerve rings the animal is able 

 to co-ordinate the action of all the umbrella muscles and perform 

 definite swimming movements. If they are cut away the umbrella 

 seems incapable of co-ordinate movement. Even more than this, 

 it has lost all its automatism, that is to say, the power of originating 

 the stimuli that bring about muscular contraction, so that it cannot 

 move of its own accord. Thus, although we have an increased 

 development of the nervous system, it is accompanied by the 

 circumscription of the power, for if Hydra be cut into pieces each is 

 capable of separate contraction and expansion. In other words, 

 the automatism is diffuse in Hydra. Throughout the animal 

 kingdom in general we find that differentiation of structure is 

 accompanied by localisation of function. 



Hydranth and Polyp. 



Superficially there is little resemblance between the hydroid 

 and medusoid type of person, and yet a closer examination shows 

 that the fundamental structure is the same in both cases. They are 

 constructed upon the same plan, and are to be regarded as homologous. 

 Suppose we imagine a polyp much shortened in its main axis, and 

 at the same time the transverse diameter much increased by the 

 expansion of the narrow rim between the hypostome and the bases 

 of the tentacles, then a slight curvature will produce a medusa-like 

 form. The rese'mblance is not complete, for, although we should 

 have comparable oral and aboral sides, tentacles, and manubrium, 

 and the same main axis of symmetry passing through mouth and 

 aboral pole, the enteron and mesoglea would differ markedly. The 

 next stage necessary would be the thickening of the mesoglea, which 

 would bring about a reduction in the large gastral cavity, otherwise 

 occupying the whole of the space between oral and aboral surfaces 

 right out to the periphery. If the mesoglea thickened much more 

 over certain areas than others it could lead to the obliteration of the 

 enteron, save in the positions occupied by the stomach and canals 

 and the formation of the entoderm lamellae, and so we should have 

 the condition of the adult medusa. Needless to say, the medusa 



