582 THE INVERTEBRATA 



plication of stone canals and pores (see p. 551). There are no am- 

 pullae, but the diameter of the canals can be varied by muscular 

 strands which traverse them. 



Two other recent crinoids may be mentioned here. Pentacriniis is 

 a deep-water form with a long, jointed stalk, bearing whorls of cirri 

 at intervals. The adult, like Antedon, breaks free and swims by waving 

 its arms, but trails its stalk behind it. Rhisocrinus (Fig. 436) has 

 a jointed stalk without cirri except at the distal end, where some 

 branching root-cirri are developed. By these the animal is per- 

 manently rooted. It is found at great depths in the Atlantic. 



EXTINCT CLASSES 

 Echinoderms belonging to several groups now extinct are 

 numerous as fossils in Palaeozoic rocks. They are all sessile by the 

 aboral side, a fact whose significance has been mentioned above 

 (p. 557). Their body wall contains an armour [theca) of plates, 

 in which mouth, anus, and madreporite can often be identified. 

 These echinoderms are usually classified with the Crinoidea as 

 Pelmatozoa, in contrast to the free members of the phylum, which 

 constitute the Eleutherozoa. 



The following are the groups referred to in the foregoing paragraph : 



Amphoridea. The most primitive echinoderms. Body sac-like, 

 its skeleton showing no food grooves or other traces of the ambulacral 

 system. Aristocystis, Ordovician. 



Carpoidea. Stalked, and with secondary bilateral symmetry owing 

 to compression upon a plane in which lie mouth, madreporite, and 

 anus. Two food grooves on the theca have been described. Sometimes 

 there are two arm-like spines at the ends of the oral edge. Trochocystis, 

 Cambrian; Placocystis, Silurian. 



Thecoidea (Edrioasteroidea). Cushion-shaped, without stalk or 

 arms. Five food grooves, provided with covering plates, radiate from 

 the mouth. Stromatocystis, Cambrian iJS'JnofZsier (Fig. 436), Ordovician. 



Cystoidea. Body sac- or vase-shaped, with stalk, and with food 

 grooves, either on the theca (epithecal) or on special ossicles (exothecal). 

 The grooves may be carried partly or wholly by hrachioles — arm-like 

 processes of the oral side of the body. The plates of the theca are 

 hexagonal, and bear pores, either in pairs {diplopores) or in diamond 

 patterns {pore-rhombs). The members of the group fall into two 

 divisions as follows : 



DiPLOPORiDA, with diplopores and epithecal grooves. Eucystis, 



Ordovician. 

 Rhombifera, with pore-rhombs and exothecal grooves. Echino- 

 sphaera, Ordovician ; Lepadocrinus, Silurian. 



Blastoidea. Highly organized forms, typically with ovoid, stalked 

 body. Certain plates of the theca (basals, radials, deltoids, lancets, etc.) 

 are uniform in arrangement throughout the group. The ambulacra 

 are bordered by rows of pinnule-like brachioles, and at the sides of 

 the ambulacral grooves run elongated internal pouches, the hydrospires, 

 which open to the exterior at the oral end by spiracles. Pentremites, 

 Carboniferous. 



