142 Introduction to Animal Morphology. 



is never contractile. Sometimes the ambulacral radii 

 extend from one pole to the other (amb. simplicia or 

 perfecta), or they may be dorsal alone (a. interrupta, 

 or if arranged in lobate rays, a. petaloidea). The 

 pores are simple or slit-like (some Clypeastridae, &c.) 

 In Spatangoids, the hinder inter-ambulacral field has 

 large symmetrically disposed spines (sternum). The 

 mouth may be polar, eccentric, circular, or valvular, 

 two-lipped (Spatangidae), and is sometimes surrounded 

 by a petaloid arrangement of pores (floscellus). The 

 peristomial membrane may have thick lips and cal- 

 careous plates(Echinoneus minor). Radial (dccurtatorcs 

 Idbionnn] and circular muscular fibres are present in the 

 lips. Teeth may be present, rudimentary (Galerites), 

 or none (Spatangidse, Echinoneus, Cassidulina). The 

 prristomial edge of the corona is often notched for the 

 v^ills when they exist. 



The anus is round or oval, and may be polar in a 

 granular plate (periproct, the morphological equiva- 

 lent of the whole dorsal surface of Asterias) in the 

 centre of the genital circlet. When eccentric, it may 

 open on the vertical side of the perisome (plcnrocysta], 

 or on the ventral side (catacysta), near the mouth, 

 midway between it and the margin, or at the margin 

 (Echinarachnius). 



The dental apparatus is conical, with its narrow end 

 directed outwards, and its base inwards, and it can easily be 

 examined in the common sea urchin ; it is suspended by 

 muscles and ligaments to the auricles, and consists of five 

 wedge-shaped jaws arranged around the pharynx. The flat 

 sides of each jaw are transversely striated and opposed to 

 each other. Each jaw consists of two segments united by 

 suture. These diverge at the base, and are separated by an 

 interspace, arched over by an inter-radial arch (Ergiinzung- 



