640 THE INVERTEBRATA 



on each side of the base of each arm (Fig. 444 A, g). The ectoderm 

 lining the bursae retains its cilia and causes currents which subserve 

 respiration. 



Ophiura, Ophiocoma, Ophiothrix, Amphiura. British genera, separ- 

 ated by relatively unimportant differences, which are chiefly evident 

 in the ossicles and spines. Amphiura is hermaphrodite and viviparous. 



B.w. 



^wv-sy. 



ext.ir.iii.i 

 or.plt. 



Fig. 446. A diagram of a section of an ophiuroid, passing through an inter- 

 radius and part of the opposite arm. B.w. body wall ; coe. coelom ; coe.' coelom 

 of the arm; coe". perioesophageal sinus; epin. epineural canal; ext.ir.m. ex- 

 ternal interradial muscle; g.rch. genital rachis lying in the aboral sinus; 

 in.ir.m. internal interradial muscle; M. mouth; w. nerve ring; n.' radial 

 nerve ; or.f. oral tube foot ; or.plt. oral plate ; Po. Polian vesicle ; ra.peh. radial 

 perihaemal canal; tth. teeth; tor. ossicle known as torus angularis; tr.m. trans- 

 verse muscle ; wv.sy. water vascular system : to the left the circumoral ring, 

 to the right the radial vessel; i, ist ambulacral ossicle, which is displaced 

 into an interradius and known as a "peristomial plate"; 2, 3, 4, 2nd to 4th 

 ambulacral ossicles which form "vertebrae"; 2', extension of first vertebra 

 towards an interradius; i, ii, iii, ist to 3rd under ("ventral") plates. 



Class ECHINOIDEA 



Globular, cushion-shaped, or discoidal Echinodermata, without 

 arms; with small abambulacral area, in which lies the madreporite; 

 ambulacral grooves covered ; tube feet ending in suckers ; numerous 

 long spines; and pedicellariae. 



The characteristic form of body of the Echinoidea is such as would 

 result if the arms of a starfish were drawn up into the body by shrink- 

 age of the aboral surface. 



We shall describe the anatomy of this group by an account of a 

 typical member of it — Echinus esculentus, a large species common in 

 Britain. This animal (cf. Fig. 447) has the shape of a sphere with one 

 side flattened, slightly polygonal in equatorial outline. In the middle 

 of the flattened side is the mouth. Under the delicate, ciliated epi- 

 dermis an armour, the shell or corona, composed of dermal plates 

 firmly sutured together, encloses most of the body, but at the two 

 poles there are leathery areas, the peristome around the mouth, and 

 the periproct in which the anus lies excentrically. The corona (Fig. 



