492 THE AXATOMY OF IXYERTEBRATED AXIMALS. 



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

 exist on the terminal enlargement. These bands of pecu- 

 liarly-modified spines are called semitoe or fascioles, /Setnitce 

 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- 

 petalous, or, when they encircle the inner terminations of 

 their ambulacra, intra2)etalous (Amjyhidotus) (Fig. 143, A, B). 



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

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

 ( Cidaris) interarabulacral, 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- 

 rlcidce. 



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 trabeculae, so that 

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



The Spatangoida present neither Auricidoe nor other in- 

 ternal processes. 



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

 beyond a c^ecal 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 mesenterv. 



In the Echinidea., 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 epip)iysis, and an 

 inferior principal portion, united together. Each alveolus 

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

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

 so as always to develop a sharp edge 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 



