DEVELOPMENT OF ECHINODERMS. 275 
The recent forms include the stalked Pentacrinus, Rhizocrinus, etc., 
and the free Comatulids, which pass through a stalked Pentacrinus 
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 BLastorpEa. 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 CystipgEa. 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 ; (4) 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 
