208 HYATT, 



the arms, the number of these, also, is determined by the 

 same character. 



LOPHOPHORIO TLEXOR. 



This muscle, extending from the oesophagus to the tips 

 of the arms, is necessarily very short in Fredericella, if it 

 exists at all ; and longer in the other genera, in propor- 

 tion to the length of the arms. 



NEURAL DIAPHRAGM. 



Sufficiently explained by synopsis. 



NERVE-MASS. 



In Fredericella, Avhere there are no arms, and the 

 breadth of the lophophore inconsiderable, it is not neces- 

 sary to have the centres of distribution widely separated, 

 and, therefore, the nerve-mass has perhaps no com- 

 missure. In Plumatella, however, with its two long 

 arms, each to be rendered capable of independent motion, 

 and the breadth of the lophophore increased, the ganglia 

 are probably separated by a commissure so as to corre- 

 spond with these changes. 



GANGLIA. 



The thin, upright, spindle-like form of the ganglia, as 

 seen from the side in Fredericella, is changed in Pluma- 

 tella to a kidney-shape, lying horizontally. This corre- 

 sponds precisely with increased width, which is given by 

 development of the arms for the accommodation of the 

 nerve-mass. 



The heart-shaped appearance of the ganglia is invari- 

 able ; because, when either the spindle or kidney-shaped 

 double upon themselves by contraction, they assume the 

 heart-shape. 



The larger size of the lophophoric nerve-trunk, the_ 

 greater length of the dorsal lophophoric nerve-branch and 

 its plication, the decrease in the ventral lophophoric 

 nerve-branch proportionally to the dorsal branchy the 

 number of tentacular nerves, the larger size of the oral 



