"ill in !.i.i:ii\ 82, DOTTED STATES NATIONAL Ml SETJM 



pinnules. He said further thai this assumption of the extension of the gonads under 

 the interradii lie chief cause oi the enlargemenl of the perisome makes it clear 



a|i\ iu yi aatulids we do qoI get a high disk in connection with the pinnule 



gap, fur. mi this stage the gonads are whollj rudimentary. 



I >r. ( iislen's explanation of the pinnule gap i- most interesting and ingenious and 

 certainly i- worthj of Berioua consideration. But the author still maintains Ids 

 belief in his <>« □ interpretation of the conditions in the early pentacrinoids. There is, 

 of course, the possibility that both of us are in error in ascribing the pinnule gaps in 

 the recent crinoids to factors lia\ ing their origin in plrj Logenetical antecedents. These 

 gaps may well he nothing more than a response to the conditions of environment with 

 no phylogenetical background at all. 



In the case "I the developing young of the comatulids the prime requisite is to 

 secure food :i- early as possible. The larger the diameter of the circle combed by the 

 fund-collecting organs, the larger is the potential food supply. So pinnules first 

 appear at the tip- of the growing arms, where it is obvious they will he most effective. 

 With the commencement of concentration of food upon the disk, resulting from the 

 action of the pinnules at the arm tips, the protection of the disk becomes increasingly 

 important, so that it would he natural to expect the tactile pinnules at the edge of the 

 disk to put in an appearance. The need for the intermediate pinnules, which are 

 either genital or supplementary tactile pinnules, or both, does not arise until later, 

 BO that their development is delayed in favor of that of the more important pinnules. 



The absence "I a varying number of basal pinnules in such types as the Atele- 

 crinidae and Bathycrinidae is probably due to the fact that, as a result of the crowding 

 of the arm bases, there i< no room for them. If present they would he without value 

 to the animal, and bo they are not developed. Such an explanation is easy to com- 

 prehend if pinnules are regarded in the light of articulated appendages, but is [ess 

 easy of comprehension if they are considered as reduced arm ramifications. 



INTERRELATIONSHIPS OF THE COMATULIDS 



Gislen's rearrangement of the comatulid groups was given in detail in Part 3, 

 p. ;-'■- '.I' '■: Hi- imt,- <.n the cMilutiun ami interrelationships of the comatulids 

 arc of much interest, for his was the first serious attempt to harmonize in detail the 

 systematic background of the recent and the fossil forms. 



JTu evolution oj certain comatulid characters. — Gislen said that originally cirri 

 were arranged in five radial columns. It was not until the suppression of the inter- 

 nodals in fche pentacrinite stem and the increased shortening of the nodals took place 

 that two alternating columns occurred, to conserve space, in each radius. Since 

 after the development of the centrodorsal the Dumber of the cirri continued to increase 

 a third column appeared, and finally the arrangement became a series of closely 

 crowded irregular alternating rows. Tin- developmental course may be traced 

 more or less distinctly in the ontogeny of certain comatulids. Centrodorsals with 



ly crowded alternating rows of cirri are therefore the most highly specialized. 



Originally the cirrus sockets had a distinct sculpture, hut this has disappeared 

 more or less completely among the younger forms. 



The cavity within the centrodorsal was originally, as all the fossil comatulids 

 show, from fairly small to \ery inconspicuous, never larger than in the corresponding 



