MONOGRAPH OF THE EXISTING CRINOIDS. 



189 



tinuously add new plates to the scries between the ventral apex and the plates 

 already formed. 



In the crinoids exactly the same formation of now plates occurs: hut there 

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

 hence those plates turn outward and as they form give rise to long arms, at first 

 biserial and later becoming uniserial, bearing the ambulacral processes on their 

 ventral surface. 



Fig. 121.— Lateral view of a specimek of Peniametrocrinus tcbercclatis from southern Japan, showino the 

 relative proportions of tue arms, rinnules, centropoi^al, and cirri. 



Nervous system. 



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

 fusion of ganglia, which becomes more marketl in the more sjjecialized types; in 

 the crabs the ventral chain is represented by a lobed ganglionic mass in the thorax 

 connocted with a mere rudiment which corresponds to the abdominal portion of 

 the cord in tho more elongate decapods. In the deca|)ods the number of fully 



