64 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery of Echinoclerma have descended and to approach most nearly 

 those pre-existing animals from which the Echinoderma were 

 Table-ease originally derived. The best British specimens are from the 

 W ll^c se ^^^^-'^'^^^ Limestone ; but other horizons are well represented 

 \q^^^^ by those from Bohemia, Eussia, ISTorth America and other 

 foreign countries. Among the older and more primitive are 

 many, such as Aristocystis (Fig. 29 a), that appear to have 

 been little more than plated sacks, without stem or arms : 



U7VUS 



go-noJooTe 

 UHxIer-pore ~ ~ 

 rrvoxdh - 



Fig. 29. — Types of Cystidea. a, Sack form with scattered pores, Aristo- 

 cystis. b, Example of Rhombifera, with food-groove skeleton slightly 

 developed, Echinosphaera. c, Example of Diploporita, Protocrinus ; 

 the brachioles are restored on the right side ; elsewhere are seen the 

 facets that supported them, d, Example of Rhombifera, with food- 

 groove skeleton highly developed, and with respiratory folds restricted 

 to three " pectini-rhombs," of which one is shown near the top of the 

 right-hand quarter ; Lepadocrinus, 



hence the name of the class, which means " sack-shaped." 

 These, however, probably all had ciliated food-grooves stretch- 

 ing from the mouth, either along fleshy tentacles or along 

 similar processes provided with a calcified support or skeleton. 

 These processes are called brachioles, and there is no reason 

 to suppose that they contained such extensions of the body- 

 cavity, of the reproductive organs, or even of the hydraulic 

 system, as occur in Crinoidea. Hence we suppose that a 



