8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. ']2 



In addition to the primary bony framework including the ossicles 

 of the crown — calyx plates, brachials and pinnulars — and stem and 

 cirri crinoids have two other skeletal systems ; one, superficial, takes 

 the form of very numerous spicules which may increase in size to 

 definite plates som.etimes mutually in contact protecting the soft 

 ventral integument ; along the ambulacral grooves, especially on the 

 pinnules, these become more regular and better developed than else- 

 where (figs. 18-26) and often form large and definite side and cover- 

 ing plates (figs. 27-31) the former lying in the perisomic wall and the 

 latter, hinged to them by ligaments, in the lappets which line the 

 ambulacral grooves and capable of being closed down over them ; the 

 other skeletal system is the internal, consisting of numerous spicules 

 and networks occurring more or less plentifully in the bands of con- 

 nective tissue traversing the body wall and in the walls of the 

 digestive canal. 



In a very large lo-armed feather-star in which side and covering 

 plates are developed there are visible externally about 600,000 distinct 

 skeletal elements each of which arises from a separate center of 

 ossification; of these about 87,000 belong to the primary and about 

 513,000 to the secondary or perisomic skeletal series. In a large 

 comasterid with no side and covering plates visible there may be as 

 many as 700,000 primary skeletal elements, while in the very small 

 antedonids the number probably never falls below 10,000. The 

 greatest of the figures, however, is insignificant when compared with 

 the number of ossicles in the larger pentacrinites where, in the recent 

 species, nearly two and one-half millions are found. These figures, 

 large as they are, must be approximately doubled when the internal 

 skeleton is taken into consideration. 



The internal structure of all the crinoids which have been studied 

 is very similar. The mouth, at the point of convergence of the 

 ambulacral grooves coming onto the disc from the arms (figs. 4, 5), 

 usually central or subcentral but often excentric or even marginal in 

 the Comasteridse (fig. 12), leads downward through the gullet into 

 the digestive tube (fig. 13) which, turning to the right, makes some- 

 what more than one complete coil (four in some of the Comasteridas), 

 the posterior portion bending upward and slightly forward to end at 

 the summit of the so-called anal cone situated on the disc between 

 two of the ambulacral groove trunks. Except for its canal-like pro- 

 longations into the arms the body cavity is filled with a mass of 

 mesenteries, bands and septa so that it is reduced to a minimum, and 

 in some cases, as in Isocrinus, has entirely disappeared. 



Within the centrodorsal in the comatulids (fig. 7) and just over the 

 summit of the column in the other types lies the chambered organ ; 



