MONOGKAPH OF THE EXISTING CRINOIDS. 



195 



(2) The secondary or perisomic skeleton. — This consists of the side and covering 

 plates, the plates of the disk (excepting the orals), and of the bracliial ])erisome, 

 and the numerous minute plates and spicules mostly lying toward tlie inner side 

 of the soft integument, orchnarily more or less iso- 

 lated, but sometimes sUghtly connected by strands 

 of connective tissue. 



The perisomic plates of the so-called secondary 

 series differ from the primary plates, among oth(>r 

 ways, in possessmg great variability, or exliibitiiig au 

 absence of fixity, in their sliape and in the method 

 and mamier of their occurrence; in other words, they 

 are directly dependent upon local mechanical condi- 

 tions, while the phylogenetically significant primary 

 plates, origmaUy just as dependent upon local me- 

 chanical conditions, have, through long existence as 

 integral units, attained a distinct entity of their own, 

 which is to a certam degree dominant over the me- 

 chanics of their immediate surroundings. 



Among the recent crinoids the interradials (and 

 the racUanal) are, through degeneration, somewhat 

 intermediate in character between this series and the 

 one preceding; the well-developed plates on thechsks 

 of the young of the various comasterids and of TJiau- 

 matocrinus wliich are resorbed before the adult contli- 

 tion is attained, also show in many ways an approach 

 to the secondary type of plate. 



There has usually been made a considerable 

 difference between primary and secondary plates, 

 but in reality no definite line of differentiation 

 exists; both types grade into each other, and the 

 primary plates are only a small phylogenctic step 

 in advance of those of the perisomic series though, 

 it must be confessed, in most cases distinct enough 

 in the adults of the recent forms. 



The more important plates of the secondarv 

 series from a systematic standpoint are the side and 

 covering plates, the jilatcs developed on the ventral 

 surface of the disk, and the plates developed on the 

 sides of the disk between the postradial series. 



(3) The visceral skeleton. — This term is used to 

 denote the numerous spicules and networks of hme- 

 stone which, as described by P. H. Carpenter and 

 others, occur more or less plentifully in the bands of 

 connective tissue that travei-se the visceral nuiss 



and in the walls of the digestive canal; these spicules grade insensibly into the 

 perisomic type, so that in effect the visceral skeleton is merely that i)art of the 

 perisomic skeleton wliich is developed within the boily. 



FlO. ia>.— LATEK.U. \1EW or THE CKOVTH 

 AND rnOXUIAL COLUMNALS or A SPECIMEK 

 OF TEUOCBINC3 SPRIXGEKI FROM TBE 

 LACCAOn'E TSL.V!n)S, SHOVnjJQ THE RE- 

 LATIONSHIPS OF THE BA9ALS, RADL\L3, 

 AKD ARSIS. 



