102 THE FAUNA OF THE DEEP SEA 



the Echini, with a disconnected apical system 

 characteristic of many caiiiozoic spatangoids ; they 

 have a sunken anal system, some of them a most 

 remarkable anal beak and a very striking pouch in 

 which the mouth is placed.' They are found only in 

 very deep water, and have no allies among the modern 

 littoral fauna. 



The genera Calveria and Pkormosoma are two of 

 the most abundant Echinoids found in deep water, and 

 they are both representatives of forms that were very 

 abundant in cretaceous times. They are remarkable 

 for the extreme flexibility of their shells. In shallow - 

 water sea-urchins the shells are composed of a great 

 number of little plates that fit so closely to one 

 another that no movement is possible between them. 

 When the animal dies all the soft tissues decay 

 and the shell remains, to be tossed about by the 

 waves until crunched or dashed to pieces. In 

 Phormosoma, however, the tiny plates of which the 

 shell is composed are freely movable on one another, 

 and when the animal is alive very considerable con- 

 tractions and expansions can take place. 



None of the modern shallow-water Echinoids 

 present this peculiarity, and it is a very interesting 

 and surprising fact that in this respect the fossils 



