284 ZOOLOGY FOR MEDICAL STUDENTS chap. 



become grouped in pairs and possessing muscles so arranged that the 

 two spines can be pulled together and function as minute forceps or 

 pincers. These organs — the pedicellariae — may be seen in large numbers 

 along the edge of the ambulacral groove and again, forming rounded 

 clumps, round the base of the ordinary spines. Their function appears 

 to be to keep the surface of the starfish free from foreign particles, 

 minute animals, etc. : they clutch these readily and pass them on. 



In the other groups of Echinoderms we find characteristic differences 

 in the skeleton. In the Ophiuroids, Crinoids, and Echinoids, it is more 

 highly developed than in the Asteroids. In the Ophiuroids the ambu- 

 lacral ossicles have become converted into compact " vertebrae " which 

 occupy a great part of the thickness of the arm and are jointed together 

 so as to form a chain freely flexible in a horizontal plane, by the move- 

 ments of which the Ophiuroid pushes itself'along. To bring about these 

 movements strong muscles pass on each side from one vertebra to the 

 next, and if removed from the water the Ophiuroid is apt to go through 

 a characteristic performance, contracting the muscles on both sides of 

 the arms at once with the result that they rupture and the arms drop 

 apart into numerous fragments, thus justifying the English name Brittle- 

 star. In the Crinoid the stalk is composed for the most part of pentagonal 

 or circular skeletal blocks arranged in a row. While at the present time 

 only a few genera and species of Crinoids survive, they flourished exceed- 

 ingly during earlier geological periods (Carboniferous, Liassic), attaining 

 to a great size and to great numbers both of species and individuals : 

 their stalk ossicles are amongst the most familiar of fossils in the lime- 

 stones of these ages. 



In the Echinoids the ossicles are large plates closely jointed together, 

 forming the well-known test or shell of the Sea-urchin. During life the 

 plates of the test though apparently fitting together edge to edge are 

 really separated by a thin layer of living tissue, this arrangement render- 

 ing possible the growth of the individual plates along their edges and 

 as a consequence the increase in size of the test as a whole. In the 

 Echinoids the spines are greatly developed, being long and usually 

 slender, and in correlation with this both tube-feet and pedicellariae are 

 correspondingly lengthened. 



In contrast with the groups mentioned the Holothurians show a 

 greatly reduced condition of the skeleton, the individual plates in the 

 body-wall being reduced to minute spicules — sometimes of beautiful and 

 characteristic shape — and thus the body-wall instead of being rigid is 

 flexible, of a tough leathery consistency. 



The alimentary canal of the starfish is comparatively short, passing 



