222 MANUAL OF ZOOLOGY. 



CHAPTER XXII. 



DISTRIBUTION OF ECHINODERMATA IN 

 SPACE AND TIME. 



DISTRIBUTION OF ECHINODERMATA IN SPACE. The Crinoidea 

 are represented by comparatively few forms in recent seas, and 

 these have mostly a very local distribution. More than one 

 hundred and fifty species of Comatula (in the wide sense) are 

 known, ranging from 82 N. lat. to Kerguelen's Land, but most 

 abundant in the tropics. Holopus is a West Indian form, and 

 the species of Pentacrinus are principally West Indian also. 

 Rhizocrinns Lofotensis occurs in the North Atlantic, and another 

 species of this genus is found in the Gulf of Mexico. Lastly, 

 the species of Bathycrimis and Hyocrinus are found at great 

 depths in the Atlantic and Pacific. 



The Asteroidea, Ophiuroidea, and Echinoidea are represented 

 in almost all seas, whether in tropical or temperate zones, ex- 

 tending their range even into high northern and southern lati- 

 tudes. They have also a wide bathymetrical range, extending 

 from between tide-marks to almost the greatest depths which 

 have yet been explored by the dredge. Some of the Sea- 

 urchins (such as Toxopneustes lividus) have the peculiar habit 

 of hollowing out cavities for themselves in the solid rock, in 

 which they spend their existence. The Holothuroidea enjoy 

 a nearly world-wide distribution ; but they have their metro- 

 polis in the Pacific Ocean, occurring abundantly on the coral- 

 reefs of the Polynesian Archipelago. One species (Holothuria 

 argus) is collected in large numbers, and is exported to China, 

 where it is regarded as a great delicacy. 



DISTRIBUTION OF ECHINODERMATA IN TIME. Numerous 

 remains of Echinodermata occur in mo.^t sedimentary rocks, 

 beginning with the Upper Cambrian rocks, and extending 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 mostly referable 

 to the saiiie epoch. The more highly organised Asteroidea 

 and Ophiuroidea commenced to be represented in the Silurian 

 period ; but the Echinoidea^ with few exceptions, have no rep- 

 resentatives earlier than the Carboniferous rocks. The fol- 

 lowing exhibits, the geological distribution of the different orders 

 of the Echinodermata in somewhat greater detail : 



i. CRINOIDEA. The pedunculate Crinoidea attained their 



