(34 GUIDE TO THE FOSSIL INVEKTEBEATE ANIMALS. 
Gallery of Echinodemia liave descended and to api)roach ino.st nearly 
tliose pre-exi.sting animals from wliich the Kchinoderma were 
Table-case originally derived. Tlie best British specimens are from tlie 
WalLcase Limestone ; but other horizijns are well represented 
13 by those from Bohemia, iius.sia, ISTortli America and other 
foreign countries. Among tlie 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 : 
1 Ttiouih' 
\h 
mou-lh. 
v/a^r - 
pore 
anus 
anus 
Fig. 29. — Types of Cystidea. a, Sack form with scattered pores, Aristo- 
cystis. b, Example of Rhombifera, with food-groove skeleton slightly 
developed, Echinosphacra. c, Example of Diploporita, Protocrinus ; 
the brachioles are restored ou 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 cLoss, which means “ sack -shaped.” 
These, hnw'ever, 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 rei)roductive organs, or even of the hydraulic 
system, as occur in Crinoidea. Hence we suppose that a 
