DE VEL OPMENT OF ECH1NODERMS. 27 5 



The recent forms include the stalked Pentacrinus, Rhizocrinus, etc., 

 and the free Comatulids, which pass through a stalked Penlacrtnus 

 stage, e.g. Antedon. 



Class EDRIOASTEROIDEA. Wholly extinct 



These extinct Pelmatozoa had a sac-like theca of an indefinite number 

 of irregular plates, with a mouth in the centre of the upper surface, 

 with at most a short stalk. Ordovician, Silurian, and Devonian. 

 " They are alone among Pelmatozoa in presenting a type of ambulacrum 

 from which the holothurian, stellerid, and echinoid types may readily 

 be derived " (F. A. Bather). 



Class BLASTOIDEA. Wholly extinct 



The Blastoids are first found in the upper Silurian, later than Cystoids 

 and Crinoids ; they had their golden age in the Carboniferous and 

 Devonian times, but then disappeared. Their body was ovate, with 

 five ambulacral areas, with each groove of which jointed pinnules were 

 associated. 



Class CYSTIDEA. Wholly extinct 



The Cystidea are first found in the Lower Silurian rocks, had their 

 golden age in Upper Silurian times, and died out in the Carboniferous 

 period. Their body was ovate or globular, sessile or shortly stalked, 

 covered with polygonal plates often irregularly arranged. 



DEVELOPMENT OF ECHINODERMS 



The ovum undergoes total segmentation, and a hollow 

 ball of cells or blastosphere results. A typical gastrula is 

 formed by invagination. 



The mesoblast has a twofold origin : (a) from " mesen- 

 chyme" cells, which immigrate from the invaginated endo- 

 derm into the segmentation cavity ; (b) from the outgrowing 

 of one or more ccelom pouches (vaso-peritoneal vesicles) 

 from the gastrula cavity or archenteron. From these 

 vesicles the body cavity and the rudiments of the water- 

 vascular system arise. 



The larva is, first of all, a slightly modified, diffusely 

 ciliated gastrula. In Holothuroids, Echinoids, Asteroids, 

 and Ophiuroids, it becomes quaintly modified by the 

 outgrowth of external processes, and the formation of 



