658 THE INVERTEBRATA 



and so reach the pinnules, where they enlarge into gonads. The genital 

 cells are dehisced by rupture when ripe. The lacunar system has an 

 oral ring and "vessels" from this to a plexus on the stomach and to 

 the lacunar tissue of the axial organ. It is doubtful whether radial 

 vessels are present. 



The water vascular ring closely surrounds the mouth, and from it 

 numerous stone canals open without madreporites to the perivisceral 

 cavity, which in turn communicates by many isolated pores, lined by 

 cilia, with the exterior. This arrangement is due to the fusion of the 

 larval axial sinus with the perivisceral cavity and subsequent multi- 

 plication of stone canals and pores (see p. 627). 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. Pentacrinus 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. Rhizocrinus (Fig. 461) has a 

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

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

 ently 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. 633). 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. 



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

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

 the mouth. Stromatocystis, Cambrian ; Edrioaster (Fig. 461), 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 brachioles — arm-like 



