SH 



PHYLUM ECHINODERMA 



Certain of them form tentacles around the mouth, and these are supphed 

 by canals coming off directly from the ring canal. 



The sexes are separate. The reproductive organs extend as tubular 

 strands from the disc along the arms, but are rarely functional except 

 in the pinnules, from each of which the elements burst out by one duct 

 in females, by one or two fine canals in males. 



The oval ciliated larva of Antedon, the only one known, is less 

 peculiar than that of other Echinoderms. 



There are about 400 living species in twelve genera, but about 1500 

 species in 200 genera are known from the rocks. The class is re- 

 presented in the Cambrian, and attained its maximum development in 

 Silurian, Devonian, and Carboniferous times. 



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 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 mesoderm has a twofold origin : {a) from " mesen- 

 chyme " cells, which immigrate from the invaginated 



