PEOPLING OF LAND AND SEA 149 



(Flabettum and others) ; the Polychelidae, a kind of 

 flattened lobster akin to the Jurassic Eryon ; among the 

 Echinodenns the fixed Crinoids, flexible sea-urchins of 

 the type of Calveria, and Pourtalesia, related to the 

 Ananchytes of the Cretaceous Period, and among the Molluscs 

 the Pleurotoma and Pholadomya. The fish also belong 

 to types best represented in fresh water, which, as we know, 

 are the oldest ; they are akin to Salmon, Pike, Eels, or Cod, 

 types with which are likewise connected a certain number of 

 pelagic fish remarkable for the numerous eyes 1 they have on the 

 sides of their body, one pair on each segment, representing the 

 remains of modified lateral sense organs. 



These archaic types disappear by degrees as we go gradually 

 deeper, and are replaced by organisms manifestly recent, 

 although specifically adapted to life in deep waters. Among 

 them the most remarkable are perhaps the Holothurians. They 

 abound along every coast. There nearly all of them are shaped 

 somewhat like a cucumber, and from this fact is derived the 

 name of one of the commonest genera, Cucumaria. Their body 

 is divided into five like parts by five rows of membranous 

 tubes ending in suckers and serving as feet. Ten more or less 

 spreading tentacles surround the mouth at one end of the 

 ' cucumber ", while the anus is situated at the other. The 

 body thus possesses an absolutely perfect radial symmetry. 

 The organisms live among the pebbles, under stones, or in the 

 fissures of rocks, and under these conditions they utilize 

 indifferently any set of feet when they move. Some, however, 

 press constantly against the ground the same portion of their 

 body which comprises three sets of ambulacral tentacles, 

 one median and two lateral (Stichoptis, Colochirus). This body 

 area is distinctly flattened, and already constitutes the 

 beginnings of a ventral sole. This attains its maximum 

 differentiation in Psoitis, which lives attached to the surface of 

 rocks, and has a mouth, definitely dorsal, surrounded by long 

 ramified tentacles. These tentacles are covered with minute 

 vibratile cilia, whose incessant pulsations direct towards the 

 animal's mouth the microscopic particles constituting its 

 food. The ventral sole becomes the rule in the ocean depths, 

 and we have already pointed out (p. 127) how it is 

 formed, and for what cause. Here we can follow all the 



1 Chauliodus. 



