4 ASTEROIDEA. 



Order n. Holothuroidea. — Body in general elongated ; skin usually soft and 

 leathery, in a few genera strengthened by calcareous or horny spines. Five avenues of 

 suckers, which divide the body into as many longitudinal, nearly equal, lobes or segments ; 

 mouth surrounded by plumose tentacula, the numbers of which are in general multiples of 

 five ; anus at the opposite extremity of the body ; digestive organs consist of a long intestine, 

 which makes some coils in passing through the body ; respiration performed by internal 

 ramified tubes, like a miniature tree ; locomotion effected by contractions and extensions of 

 the body, and by rows of tubular suckers, similar to those in the Star-fishes and Sea-urchins. 

 The softness of their naked integument prevents their preservation in the stratified rocks. 

 We know none in a fossil state. 



Type. Cucmnaria frondosa. Griiner. 



Order in. Echinoidea. — Body spheroidal, oval, or depressed, without arms ; furnished 

 with a distinct mouth, sometimes armed, which is always below, and an anus which 

 occupies different positions. Body enclosed in a shell or test, composed of twenty columns 

 of calcareous plates, and ten rows of holes for the passage of retractile tubular suckers ; 

 the surface of the test is studded with tubercles, which possess, jointed with them, moveable 

 spines, of various sizes and forms in the different families and genera : at the summit of 

 the test is the apical disc, composed of five genital plates, perforated for the passage of the 

 ovarial and seminal tubes, and five ocular plates for lodging the five eyes. The intestine 

 winds round the shell, attached by a mesentery, the surface of which, as well as the mem- 

 brane lining of the test, is covered with vibratile cilia. 



Type. The common Sea-urchin, Echinus sphcera. Midler. 



The Echinoidea are represented by one family in the Palaeozoic rocks, and by 

 numerous families in the Mesozoic and Tertiary rocks, several of which characterise these 

 great periods of geological time. They likewise abound in our present seas. 



Order iv. Asteroidea. — Body stelliform, depressed, provided with five or more lobes 

 or hollow arms, which are a continuation of the body, and contain prolongations of the 

 viscera ; the mouth, which is always below and central, serves likewise as an anus ; rows 

 of retractile tubular suckers occupy the centre of the rays. Skeleton complicated, com- 

 posed of numerous solid calcareous pieces, variable as to number, size, and disposition ; 

 skin coriaceous, studded with calcareous spines of various forms ; a madreporiform plate on 

 the upper surface, near the angle between two rays ; eyes placed at the extremity of the 

 rays ; reptation performed by the tubular suckers. 



Type. The common Star-fish, Uraster rubens. Linnaeus. 



This order is represented in the Silurian rocks by two genera. The Oolitic, Cretaceous, 

 and Tertiary rocks contain many extinct forms. The existing species are very abundant 

 in all the present seas. 



