492 THE ANATOMY OF INVERTEBRATED ANIMALS. 



cilia cover the shaft of the spine, while no such structures 

 exist on the terminal enlargement. These bands of pecu- 

 liarly-modified spines are called semitce or fascioles. Semitm 

 lie beneath and surround the anus in some genera, and are 

 called subanal and circumanal ; others surround the outer 

 extremities of the petaloid ambulacra, and are termed peri- 

 petaloiiSj or, when they encircle the inner terminations of 

 their ambulacra, intrapetalous [Ampjhidotus) (Fig. 143, A, B). 



If we turn to the interior of the shell of the Ecliini- 

 dea^ we find in the Echinoida that ambulacral, or sometimes 

 (Cidaris) interambulacral, plates of the oral margin of the 

 corona are produced into five perpendicular perforated pro- 

 cesses, which arch over the ambulacra, and are called the au- 

 riculm. 



Besides these, processes are developed from the ambula- 

 cral plates in Cidaris which form a sort of wall on each side 

 of the ambulacral canal, but do not arch over it. In Clypeas- 

 ter^ similar processes form complete arches; and in the flat- 

 tened Clypeastroid Scutella^ the oral and apical walls of the 

 corona are united together by calcareous trabeculse, so that 

 the cavity of the body is restricted to a very small space. 



The Spatangoida present neither Auriculce nor other in- 

 ternal processes. 



In the Echinidea^ the oesophagus is usually distinct, but, 

 beyond a caecal diverticulum in some cases, there is no further 

 differentiation of the alimentary canal, which is disposed spi- 

 rally around the walls of the corona, and attached thereto by 

 a mesentery. 



In the EcJiinidea^ the oral skeleton attains its highest 

 development in the so-called " Aristotle's lantern " of the 

 Sea-urchins (Fig. 144, B, C, D). 



This apparatus consists of five hollow, wedge-shaped, cal- 

 careous pieces — the alveoli (Fig. 144, B, a^ — each of which is 

 composed of two halves united together in the middle line, 

 while each half again consists of a superior epiphysis, and an 

 inferior principal portion, united together. Each alveolus 

 serves as the socket for a long tooth (c), shaped somewhat 

 like the incisor of a Rodent, harder externally than internally, 

 so as always to develop a sharp ^d.^Q, with wear. The tooth 

 constantly grows from its upper extremity, while its lower 

 half becomes united with the wall of the alveolus. The five 

 alveoli, if fitted together, form a cone, the applied surfaces of 

 which are united by strong transverse muscular fibres, while 

 superiorly, the epiphyses of each pair of alveoli are connected 



