ECHINODERINIATA — STARFISHES 



length are ten vertical partition 

 plates ; in each of the ten compart- 

 ments thus formed are lodged two 

 arms, which unlike Pentacrinus never 

 extend upward beyond the calyx. 



1. Sketch side view, noting calyx, 

 vertical partitions, arms if preserved. 



2. In what conspicuous feature 

 does this genus differ from most other 

 crinoids ? 



CLASS D, ASTEROIDEA (STARFISHES) 



Body free, star-shaped, with a cen- 

 tral disk and hollow arms (usually 

 five in number), each containing a 

 prolongation of the organs of the 

 body. Movement is by means of 

 the tube-feet which project between 

 the plates from the five ambulacral 

 grooves radiating from the mouth. 



Asteroidea range from the Cam- 

 brian to the present. Living ex- 

 amples include Aster ias vulgaris and 

 Asterias forbesi (p. 149), the common 

 starfishes of the Atlantic coast, the 

 former north and the latter south of 

 Cape Cod. There are many tropical 

 species. 



Derivation of name. — Greek aster, 

 star, -h eidos {old), form, in allusion 

 to the star-shaped body. 



Paleaster (Fig. 64). 



Ordovician to Mississippian. 



Differs from Asterias in having 



the two rows of ambulacral plates 



Fig. 63. — Eucaly ptocrinus cras- 

 sus Hall, from the Middle 

 Silurian of Indiana, a., arms ; 

 c, calyx or cup; h. f., hold 

 fasts, the rootlike basal part 

 for attachment to the ground ; 

 St., stalk. Slightly reduced. 

 (From Hall.) 



