138 MANUAL OP ZOOLOGY. 



body is provided with, powerful longitudinal and circular 

 muscles. 



In Synapta there is no ambulacral system, nor respiratory 

 tree. Locomotion is effected by means of little, anchor-shaped, 

 calcareous spicules, placed upon little papillae of the integu- 

 ment. 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. Holotkurida. 



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. No ambulacral system. 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 Gomatula 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. Bhizocrinus 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 Asteroidea, Opliiuroidea, 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 re- 

 mains of Ecliinodermata occur in most sedimentary rocks, 

 beginning with the base of the Lower Silurian Rocks, and ex- 

 tending up to the recent period. The two orders Cystoidea 

 and Blastoidea, which are the most lowly organised of the 

 entire class, are exclusively Palaeozoic ; and the Crinoidea are 



