/OOLOGY FOR MEDICAL STUDENTS CHAP. 



become grouped in pairs and possessing muscles so arranged that the 



i.e pulled together and function as minute forceps or 



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



of the ambulacral groove and again, forming rounded 



.-lumps, 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 

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

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

 lacral os>iclrs have heroine converted into compact "vertebrae" which 

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



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



01 which the Ophiuroid pushes itself along. To bring about these 

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



:ul if removed from the water the Ophiuroid is apt to go through 

 i- -t eristic performance, contracting the muscles on both sides of 

 is 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 leu penera and species of Crinoids survive, they flourished exceed- 

 ;riim earlier geological periods (Carboniferous, Liassic), attaining 

 size and to ureat numbers both of species and individuals : 

 ilk ossicles are amongst the most familiar of fossils in the lime- 

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

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

 '! the test though apparently fitting together edge to edge are 

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

 th of the individual plates along their edges and 

 'icnce the increase in size of the test as a whole. In the 

 I- 1 hin- pines are greatly developed, being long and usually 



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



dingl) lengthened. 



In contrast with the groups mentioned the Holothurians show a 



irdiurd condition of the skeleton, the individual plates in the 



body-wall bring reduced to minute spicules sometimes of beautiful and 



UK! thus the body-wall instead of being rigid is 



ithcry consistency. 



canal of the starfish is comparatively short, passing 



