ANNULOIDA : ECHINODERMATA. 155 



mentary skeleton. Many of the Sea-cucumbers, in fact, are 

 endowed with such high contractility that they can eject their 

 internal organs entirely, if injured or alarmed. 



In Synapta there is no ambulacral system of tube-feet, nor 

 respiratory tree. Locomotion is effected by means of little, 

 anchor-shaped, calcareous spicules, placed upon little papillae 

 of the integument. Respiration is effected in the abdominal 

 cavity, into which the water is admitted by five openings 

 between the tentacles. 



The order Holothuroidea may be divided into the following 

 two families : 



Family I. HolotJmridce. 



Body free, cylindrical, with a coriaceous integument containing 

 scattered calcareous particles. An ambulacral system always, and a 

 respiratory tree usually, present. 



Family II. Synaptida. 



Body free, covered with a coriaceous, sometimes soft, integument, 

 containing minute, anchor-shaped spicules, by means of which the 

 animal moves. The ambulacral system rudimentary, not giving rise 

 to tube-feet, and not connected with locomotion. A respiratory tree 

 sometimes present, sometimes absent. 



CHAPTER XXIII. 



DISTRIBUTION OF ECHINODERMATA IN SPACE 

 AND TIME. 



DISTRIBUTION OF ECHINODERMATA IN SPACE. The Crinoidea 

 are represented by very few forms in recent seas, and these 

 have a very local distribution. The Comatula is the commonest, 

 and occurs abundantly on some parts of our coasts. The 

 Pentacrinus Caput-Medusce is exclusively confined, as far as is 

 known, to the Caribbean Sea. Rhizocrinus Lofotensis has been 

 dredged on the coast of Norway, and a form believed to be 

 the same has been found in the Gulf of Mexico. 



The Astcroidea, Ophiuroidea, and Echinoidea are represented 

 in almost all seas, whether in tropical or temperate zones; 

 some occurring very far north. The Holothuroidea have their 

 metropolis in the Pacific Ocean, occurring abundantly on the 

 coral-reefs of the Polynesian Archipelago. One species is col- 

 lected in large numbers, and is exported to China, where it is 

 regarded as a great delicacy. 



DISTRIBUTION OF ECHINODERMATA IN TIME. Numerous 



