MONOGRAPH OF THE EXISTING CRINOIDS. 



189 



tinuously add new plates to the series between the ventral apex and tho plates 

 already formed. 



In tho crinoids exactly tho same formation of new plates occurs; but then- 

 is no drawing down of the radial ninbulacral processes toward tho dorsal pole; 

 honce these plates turn outward and as they form give rise to Ion;; arms, at first 

 biserial and later becoming uniserial, bearing tho ambulacra! processes on their 

 ventral surface. 



Fio. 121. LATERAL VIEW <>r A SPECIMEN OF PEXTAMETRUCKIXCS TVBERCULATUS FROM SOUTHERN JAPAN, SHOWING THE 



RELATIVE PROPORTIONS OF THE ARMS, PISNL'LES. < ENTRODORSAL, ASD CIRRI. 



Nervous system. 



In the nervous system of tho arthropods there is always a certain amount of 

 fusion of ganglia, which becomes more marked in the more specialized typas; in 

 the crabs the ventral chain is represented by a lobed ganglionic mas-, in the thorax 

 connected with a mere rudiment which corresponds to tlm abdominal portion of 

 the cord in the more elongate decapods. In tho decapods tho number of fully 



