echinodermata. 157 



Order 2. — Echinoidea. 



The body of the Echinus is enclosed in a firm hollow shell, formed 

 of polygonal plates united by sutures in twenty vertical series, arranged 

 in ten pairs. This shell (or skeleton more properly, for it is not strictly 

 external) has a spherical, oval, pentagonal, hemispherical, conoidal, or 

 discoidal form. There are six areas : ambulacral, interambulacral, 

 ocular, genital, oral and anal. Ten broad bands (interambulacral) alter- 

 nate with ten narrow bands (ambulacral). The former are studded with 

 tubercles 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 Linnaeus. They are traversed by numerous pores for the ex- 

 sertion of tubular feet or suckers, which are used for locomotion. 

 At the summit of the test is a disk composed of five genital plates, 

 and five ocular plates notched for lodging the eyes. The mouth 

 has five sharp angular teeth, tipped with enamel. These representatives 

 of molars and incisors move upon each other ; and the entire pyramidal 

 mass can be protruded and retracted. This description of the Echini 

 will not accurately apply to the irregular forms. In these the ambula- 

 cra are often only half of the typical number, and often only partial in 

 their extent. In many, too, the oral and anal openings lose their central 

 position, and approach the margin. The mouth, also, is sometimes 

 toothless. The mouth is central and dental in the Cidaridse, Echinidse, 

 Salenidse, Galeritidse and £ Clypeasteridse ; it is more or less eccentral 

 and edentulous in the Echinonidss, Collyritidse, Echinolampidee and 

 Spatangidse. 



The fossils of this Order are usually divided into three groups : 

 Cidaridse, or Turban-shaped Echini, Clypeasteridse or Buckler-shaped 

 Echini, and Spatangidse or Heart-shaped Echini. These include 71 

 genera and about 700 species. They are rare in the Palaeozoic — the oldest 

 occurring in the Upper Silurian — and attained their maximum in the 

 Oolite and Cretaceous strata. The regular forms appeared first ; the 

 elongated forms are more recent. The geological history of Echini 

 places them between the Crinoids and Star-fishes. The Crinoids abound- 

 ed in the Palaeozoic seas, but are now reduced to one or two species; 

 the Star-fishes began feebly in the Trias, but now the seas of all lati- 

 tudes swarm with them ; while the Echini increase to the Cretaceous, 

 and decline from it 



