THE ECHINOIDEA 



289 



of twenty pieces. The largest of these are the five pyramids (Fig. 

 VI. p) ; they are shaped like the sectors of a cone, being pointed 

 at one end, having two flat sides, and with a curved outer margin. 

 The flat sides are marked by transverse ridges, which serve for the 

 attachment of the muscles that bind the pyramids to one another. 

 The pyramids are hollow, and in each of them lies a long, curved, 

 keeled tooth, the hard point of which projects through a hole 

 at the pointed lower 

 end of the pyramid. 

 Above the suture be- 

 tween a pair of pyra- 

 mids rests a short, thick 

 bar known as the brace ; 

 above the brace (b) is 

 a curved bifid process 

 or " compass " (cp). 



Round the inside of 

 the peristome is a hard 

 raised rim (Fig. VII. 

 p.r) which rises into an 

 arch over each of the 

 ambulacra. This is the 

 " perignathic girdle," 

 and to it are attached 

 the muscles which work 

 the masticatory appara- 



tUSj One Set Of muscles 



ni-ta^ViPrl tn thp Tnrramirl 

 atta( ie Pyrami I 



pulls the jaws apart ; 



other sets, attached to the braces and compasses, pull them down- 



ward and drive the teeth together. 



The remaining skeletal structures are small and unimportant, 

 and consist only of scattered calcareous plates and spicules which 

 are distributed through the tissues. The principal of these are 

 the " rosettes " or " pellions " (rings of plates which support the 

 suckers), and spicules in the stems of the podia. 



The Internal anatomy of the Echinus may be most conveniently 

 studied by the removal of the upper half of the test. The five 

 ovaries will then be seen lying in the upper parts of the inter- 

 radia, and the great coiled intestine occupying most of the 

 interior. The main features in the internal anatomy are shown 

 in Fig. VII. 



The Alimentary Canal begins with a mouth (mt) situated at 

 the centre of the peristomial membrane (pst) ; from the mouth the 

 muscular pharynx (ph) passes upwards through the central tube 

 of the masticatory apparatus. The oesophagus (oe) runs horizon- 



'9 



Fl(J> VJ< 



A pyramid 



rom ell GO > P> Pyr 

 aml e> its e pjph ys is ; t, tooth ; b, brace ; cp, compass. 



Dental apparatus of Echinus csculentus, L. 

 seen from tl)e side W' and from behilld GO > P> Pyramid; 



