106 PANAMIC DEEP SEA ECHINI. 



employed in describing the " pores, spines, tubercles, the mouth slits, the 

 lining of the buccal membrane with larger or smaller plates, and the 

 calycinal area. . . . But most frequently they (these structures) are so 

 relative that it is exceedingly difficult or impossible by means of these 

 structures to decide whether a specimen in hand belongs to one species or 

 another. ... It may be simply irritating to read the descriptions of 

 these in different species that are to be compared." To remedy this 

 uncertainty, Dr. Mortensen planned what he modestly calls "a profound 

 and careful attempt at penetrating into the mysteries of the relationship 

 of the Echinoids," based upon a study of the pedicellariae. 



It is undoubtedly true that nearly all the characters which have been 

 employed to distinguish species vary with age, and the same is the case with 

 pedicellariae. They form no exception, and do not appear full}' fledged in 

 the embryos and young specimens, in spite of Dr. Mortensen's statement 

 to the contrary; though he acknowledges^ that ''there is in literature next 

 to no more exact accounts of the development of the pedicellariae of 

 Echinoids." 



Certainly before making such a sweeping use of the minute and often 

 infinitesimal characters supplied by pedicellariae for classification, it would 

 have been instructive to trace the development of the several kinds of 

 pedicellariae, and obtain some data regarding the extent and nature of the 

 variation of pedicellariae during their growth. The only addition made by 

 Dr. Mortensen to our knowledge of the development of pedicellariae is 

 shown on Figs. 15, 24, 30, PI. XII, of the " Ingolf " Echinoidea. giving three 

 stages of a triphyllous pedicellaria of Pkormosoma placenta. As long as we 

 know so little regarding the nature of the relations of the large and the 

 small pedicellariae of the same kind to one another, it seems useless to 

 speculate " on the improbability ... of the rearrangements " which must 

 " take place in the calcareous mass to make a small fully formed pedi- 

 cellaria become a larger one." Every student of Echini is fully aware 

 of the immense amount of resorption and rearrangement constantly taking 

 place in the actinal and abactinal parts of the coronal plates, in the inter- 

 ambulacral areas, and in the actinal and abactinal systems — changes that 

 are far greater than those referred to above can be. 



No one has ever questioned that it is desirable to employ in the defini- 

 tion of systematic characters all the data possible from whatever source, which 



* MorteiiscD, 1. c, p. 5. 



