BIONOMIC CHARACTERS OF VERTEBRATES 1033 



thousands of Strongylocentrotus drobachiensis are exposed at very 

 low tides, lying among stones and covered vv^ith fragments of shells 

 and small pebbles. The Echinoidea delight in a sandy bottom, from 

 which they are brought up in vast numbers at each haul of the 

 dredge. Some species prefer fine mud, in which they are often 

 buried to some extent. When living on rocks they commonly ex- 

 cavate holes for themselves, and even the solid granite has been 

 known to be thus attacked by the animal. If corners and crannies 

 are available, these are often occupied by the animal in preference 

 to the drilled hole. 



The larva of echinoids is known as a pluteus, and is a mero- 

 planktonic, bilaterally symmetrical, usually more or less ciliated 

 organism, with a number of processes or arms. It is often carried 

 by marine currents to great distances, remaining in some cases 

 afloat for several weeks before settling down. 



HoLOTHUROiDKA. The holothurians, like the echinoids, are ma- 

 rine benthonic organisms, but their habit of life is often more 

 sedentary than vagrant, the animals being buried in the sand and 

 mud, though never attached. Their bathymetric range is from the 

 shore zone, where they may be dug out of the sand at low tide, to 

 the depths of the abyssal district. Sandy or muddy bottom is usu- 

 ally preferred by these animals, though many live among coarse 

 blocks, and vast numbers occur among the coral masses of every 

 coral reef. The ciliated larva, or auricularia, of the holothurians is 

 a mero-planktonic organism with definite mouth and anal opening. 



From the fact that only isolated plates occur in the skin of 

 the holothurians, they do not constitute any important part of 

 marine deposits. 



X. Protochordata. 



These animals are all marine and unknown in the fossil state. 



XI. Vertebrata. 



OsTRAcoDERMA. These extinct fish-like animals appear to have 

 led primarily a fluviatile and lacustrine existence, if we may judge 

 by the strata in which they occur. The earliest remains are frag- 

 ments from an Upper Ordovicic sandstone (Harding sandstone) of 

 Colorado, which was probably deposited as a terrestrial sediment 

 and subsequently in part reworked by the advancing sea. The late 

 Siluric transition beds of Great Britain and eastern North America, 



