156 RADIATA. 



' CLASS I.— ECHINODERMATA. 



This Class is the true type of the Sub-kingdom. In it we first 

 recognize strikingly the radiate characters. The name was given by 

 Bruguieres, because many of the species are spine-clad. They are, how- 

 ever associated together by another and more general tegumentary 

 character; the skin is almost always perforated by minute foramina, 

 through which tubular feet can be protruded and retracted, constituting 

 the common organs of adhesion and locomotion. No Class of the Animal 

 Kingdom more clearly exhibits a gradation of structure. For while 

 some are rooted to the sea-bottom and thus resemble the Polyps, others 

 have true rayed forms clothed in prickly armor ; and these conduct us 

 to soft, elongated organisms that mimic the Molluscs, or seem to stand 

 on the lowest step of the Articulate division. The Echinoderms are all 

 marine. "While the soft-bodied Radiates float in the open sea ; to the 

 shores and fathomable depths are limited the better defended Class as 

 being better able to bear the brunt of the ceaseless conflict between 

 land and water. Unlike the Acaleph and Polyp, the Echinoderm has 

 its organs distinct from the walls of the body. The stomach is a loose 

 bag suspended in the general cavity, with intestine appendages. The 

 nervous system consists of a chord, usually pentagonal, surrounding the 

 pharynx, with branching filaments. The mouth is usually central and 

 surrounded by a circle of tentacles. In the Crinoids, the arms are near 

 the mouth upon the Ventral surface; in the Star-fishes and regular Sea- 

 urchins, it is exactly opposite the mouth ; in the Holothurioids, it is at 

 the posterior end of the body. 



The Class has been represented in all geological periods, but reached 

 its maximum development in the Mesozoic Age. " More difficult of 

 study than shells (says Professor Owen), and less uniformly present in 

 all strata, ihe enduring remains of Echinoderms and Corals are unsur- 

 passed in beauty of form and structure, and in the value of the evidence 

 they afford." 



Order 1. — Holothtjrioidea. 



These vermiform Echinoderms, vulgarly called " Sea-cucumbers," 

 constitute the highest Order of the Class. They may be compared to 

 Sea-urchins deprived of their spines, with the shell softened, and elon- 

 gated by separation of the anus and mouth. The body is either cylin- 

 drical or pentagonal, with scattered patches of calcareous deposit. 



The living Holothurioids are often found beneath stones at very low 

 water. The only examples of fossilization are small fragments from the 

 Upper Oolite of Bavaria, the Chalk of Warminster, and the northern 

 Drift of Bute, Scotland. 



