No. 560] 



CAMBRIAN HOLOTHUR1ANS 



491 



whether it possesses a stalk or not; a soft and flabby, 

 more or less shiny, tubular object, with or without body 

 processes, is at once identified as a holothurian, no mat- 

 ter whether tentacles are visible or not, and quite regard- 

 less of its symmetry. In deep-sea work one soon gets to 

 know the representatives of the various phyla by char- 

 acters never mentioned in systematic treatises, and never 

 even dreamed of by the laboratory student ; yet the iden- 

 tification by these characters after practise is quite as 

 sure as the identification by the features of real classi- 

 ficatory significance. 



The identification of many fossils calls for essentially 

 the same mental processes as the rapid identification of 

 animals brought up by the dredge ; one must be prepared 

 to grasp at once the salient correlative features if the 

 fundamental characters are obscured. Unfortunately 

 this method of work has often yielded deplorable results 

 when applied by paleontologists unacquainted with the 

 practical side of the work of the marine biologist; but 

 this is no reason why it should not lead to perfectly re- 

 liable conclusions when logically applied. 



The chief of the correlative characters in any group of 

 animals is the general body form taken in connection with 

 the size. Thus in differentiating echinoderms from other 

 organisms at sea we rely entirely upon size and shape; 

 there is no time to look for radial symmetry ; we probably 

 take this in subconsciously, though it may be to a large 

 extent mentally ignored. 



When any member of a group of animals adopts a mode 

 of life entirely different from that of all the other mem- 

 bers of the same group we must be prepared to encounter 

 extraordinary, sudden and unexpected changes in its 

 organization which are not connected with the more usual 

 type of organization by any intermediates; and it must 

 be remembered that suoh changes affect first of all the 

 general body form. Among such animals we almost 

 always find "the group characters developed in a most 

 erratic manner; some structures will be very highly spe- 



