ECHINOIDEA. II. 



specimens differ essentially ' in structure from those of the grown ones my positive statement, founded 

 on direct observations, that they are essentially alike must be accepted; by words alone it is not 

 refuted, even if it be the words of an authority so famous as Professor Agassiz. 



What most astonishes me in Professor Agassiz' objections against the systematic use of the 

 pedicellarise is his disbelief in my account of the development of the pedicellariae. The whole matter 

 seemed me so clear and its correctness beyond doubt that I did not find it necessary to figure the 

 different developmental stages of all the different pedicellarise in all the species. I might have filled 

 several plates with figures of developmental stages of pedicellarise. I have stated already in Part I. 

 p. 6 that I have found such stages of development in most of the species I have examined, and 

 this holds good also for those Echinoids, which I have studied since then. When Professor Agassiz 

 states that the only addition made by me to the knowledge of the development of pedicellarise is the 

 development of a triphyllous pedicellaria of Phormosoina placenta, he has probably overlooked this 

 remark as well as my figures of the developmental stages of a tridentate pedicellaria of Phormosoma 

 placenta. Indeed, in spite of Professor Agassiz' doubt of the correctness of my view of the mode of 

 development of the pedicellarise, I do not find it necessary to give more figures thereof. I think no- 

 body will follow the famous author in the belief that small pedicellariae are gradually, through most 

 intricate processes, transformed into large ones, a belief which is sustained by no facts, against my 

 demonstration that the pedicellariae develop at once to their final size. The reabsorption and rearrange- 

 ment constantly taking place in the test can in no way be compared with the rearrangements that 

 would be necessary for transforming a small, fully formed pedicellaria to a larger one. The changes 

 in the test can ail easily be understood as caused by the processes of absorption in some places and 

 apposition in others, but by mere apposition a valve of a small tridentate pedicellaria with fully 

 formed, even more or less decorated edges, could never get the form of a valve of a large tridentate 

 pedicellaria. Even to suppose a process of intussusception would not help, the calcareous valves not 

 being of a plastic matter like a plant-cell, but much more like some kind of crystalline structure. 



Regarding the relation of pedicellarise to the fossil forms Professor Agassiz remarks (p. 107): 

 Dr. Mortensen does not fail to perceive that pedicellarise are not likely to be of frequent use in the 

 determination of fossil forms, and for that reason condemns the classification of all fossil forms, and, 

 in passing, of the Irregular Echinoids . On this theme I have said (p. 8), after mentioning the descrip- 

 tion of the pedicellaria' of Pelanechinus corattiiius by Groom and suggesting the possibility of also 

 finding pedicellariae in well preserved specimens of other fossil Echinoids: Of course, however, it will 

 always be a rare thing - - generally we have here to be content with the tests (and the spines). These 

 structures also often give excellent characters, but they are far from being always reliable. The former 

 great incertainty in the determination of the recent forms of regular Echinoids (and I think it is not 

 much better with regard to the irregular ones) may be taken to imply that there cannot be any great 

 certainty in the classification of the fossil forms either. - - It seems to me that these few remarks are 

 indeed very moderate and can not be said to condemn the classification of all fossil forms? ; on the 

 other hand, the fact that in all the families treated in Part I the pedicellarise are of so great 



1 In Echinus the globiferous pedicellariae appear to have the blade generally somewhat more open in young speci- 

 mens than in the grown ones, as is pointed out by Doderlein. (Op. cit. p. 2ir.) 



