136 ECHINODERMATA. 



CLASS HOLOTHUROIDEA. 



These vermifoi-in ecliinoderms, vulgarly called " Sea-cncuni- 

 bers," constitute the highest order of the class. They may be 

 compared to Sea-urchins deprived of the spines, M'ith the shell 

 softened, and elongated by separation of the anus and mouth. 

 The body is either cylindrical or pentagonal, with scattered cal- 

 careous spicules or imbricated scales. 



The only examples of fossilization are small fragments of der- 

 mal ossifications from the Upper Oolite of Bavaria, the Chalk of 

 Warminster, and the noi'thern drift of Bute, Scotland. 



CLASS ECHINOIDEA. 



The body of the Echinus or " Sea-urchin " is enclosed in a 

 firm hollow shell, formed of polygonal plates united by sutures 

 in twenty vertical series, arranged in ten pairs. This shell has 

 a spherical, oval, pentagonal, hemispherical, conoidal or discoidal 

 form. Five broad zones (interambulacral) altei'nate with five 

 narrow zones (ambnlacral.) The former are studded with tub- 

 ercles bearing spines which articulate by a ball and socket joint. 

 The latter have a few smaller tubercles and spines, or none at 

 all, and appear like " walks " through the spinous tracks — 

 whence the name given by Linn sens. They are traversed by 

 numerous pores for the exsertion of suckers or '' tube-feet," 

 which are used for locomotion, and which are appendages of the 

 water-vascular system. At the summit of the test is a disk com- 

 posed of five genital ])lates, and five ocular plates notched for 

 lodging the eyes. One of the genital plates, larger than the 

 others and perforated, is the entrance of the ambnlacral or water- 

 vascular system. Tlie mouth has five sharp angular teeth, 

 tipped with enamel. These representatives of molars and in- 

 cisors move upon each other; and the entire pyramidal mass can 



