ECHINODERMATA. 



31 



downwards and surrounding the ven- 

 tral disk. Border of the mouth 

 formed by a prominent membranous 

 tube. 



Genus.'). KM HIM s. Body supported 

 on a jointed stem. (With one ex- 

 ception the species are all fossil.) 

 Order 1 1. Will Ml). I. 

 Body '/lolmlar or ovoid, without rays; skin 

 containing a calcareous shell ; amis distinct. 

 a. Ki'fiiiluriii. Mouth and amis diametri- 

 cally opposite in the centre of the ventral 

 and dorsal surface respectively. 



(ieiinsl. ECHINUS, (fig*. 33 vol. i. 



10-19.) 



Genus 2. CIDARITES. 



li. Meioitoma. Mouth in the centre, anus 

 eccentric. 



Genus 



dorsal. J 



i . Plagyottona. Mouth and anus both ec- 

 centric. 



Genus 10. ANANCIIITES. 

 ( ienus 11. SPATANGVS. 

 Order III. HOLOTBURIA 

 Body oblong, (Jig. 34 vol. i.) coriaceous, with 

 the anus (&) and mouth (a) at its opposite 

 extremities. Mouth surrounded with retrac- 

 tile, branched tentacula (<). Organ of re- 

 spiration a ramified tube (/',./', ,f)> placed 

 within the body and opening at the anus. 

 Genus. HOI.OTHURIA. (Jig. 34 p. 109, 



vol. i. and Jig. 20.) 



The genera Axtrrias, Echinus, and Holo- 

 Iliiiriti are those in which the internal struc- 

 ture has been most frequently and fully inves- 

 tigated; they are therefore usually selected' as 

 the leading examples in anatomical descriptions 

 <if the Kcliinodermata, the peculiarities of other 

 yeneia h. in..; mentioned in so far as they have 

 been satisfactorily ascertained, and are of suffi- 

 cient importance to demand special notice. 



1. Internments. An incision made through 

 the tough skin of the star-fish or shell of the 

 sea-urchin, lays open the internal cavity of 

 the body in which the viscera lie; so that in 

 these animals the integuments in a great mea- 

 sure constitute the parietes of the body, there 

 hem.; little else except the peritoneum or lining 

 membrane of the visceral cavity which is spread 

 over their internal surface. In the llolothuria 

 there are muscles of considerable thickness be- 

 neath the skin. The integuments of the former 

 animals contain imbedded pieces of calcareous 

 substance, which constitute a kind of cutaneous 

 skeleton. In the latter there is merely a calca- 



i'H4 surrounding the mouth. 

 a In tin .hi, i ins the integuments consist of, 



1st, a tough coriaceous membrane, with por- 

 tions ol calcareous substance imbedded in it, 

 or at least connected by it ; 2d, asofter external 

 membrane ; 3d, various appendages. The 

 eahaieons pieces form inferiorly a ring round 

 tli. month and a series ol' transverse segments 

 (from <i to A,Jig.T; C, fig. 22,) placed in m 

 J-'/g 7. 



Inferior oiew of Aiteriai rubem: at A part of the 

 feet M removed, 



sion along the floor of each ray. The first of these 

 segments is connected with the ring ; they de- 

 crease in size as they approach the point or 

 distal end of the ray, and openings are left 

 between them for the passage of the feet. In 

 the Asterias rubens, which has five rays, the 

 central ring consists of ten larger and five 

 smaller pieces, the former disposed in pairs 

 opposite the commencement of the rays, the 

 latter corresponding to the angles between the 

 rays. The segments of the rays are symme- 

 trical ; in the species mentioned they consist 

 of two oblong pieces (, Jig. 8), united in the 

 Fig. 8. 



Sfction of a ray of Asteritu rubtm, dotting the 

 \rratiyemCHt of the MMmMffcMi 



