80 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



at the termination, and finally in the stem they do not develop beneath the 

 highest segment, but at the upper end, the centrodorsal being the last to 

 develop. •» 



Joh. Walther '^ regards the oral tentacles as embryonal pinnules, homol- 

 ogous with the true arm-pinnules, and then arrives at the conclusion that 

 "die Arme entshehen unter die Pinnulse, nicht die Pinnute auf den Armen." 

 There is no reason to enter on a critical discussion of this and the other 

 surprising statements of this author (interradial position of the radials, 

 horizontal rotation of 36" of the main axis of Antedon, etc.), deductions 

 from entirely false premises. The indisputable fact that the oral tentacles 

 have nothing to do with the pinnules, but are homologous with the tentacles 

 of arms and pinnules, the priiliary radial tentacle especially being the homol- 

 ogon of the terminal tentacle of other Echinoderms, alone makes away with 

 these results of the said author's reflections. 



Some very curious ideas as to the true nature of pinnules have been set 

 forth. A. H. Clark, in his Monograph of the Existing Crinoids (p. 272) states: 



"Morphologically the first two segments of the pinnules are merely atropliied 

 brachials, while the remaining portion of the pinnules, including the third and suc- 

 ceeding segments, is merely a tentacular process exactly comparable to the cirri, 

 but carrying ambulacral structures on its ventral side. . . . Each brachial originates 

 as, and is fundamentally, an axillary; one of the two derivatives from this axillary, 

 after the formation of two ossicles, which are united to each other just as are the 

 paired ossicles of the division series, abruptly ceases its development, while the 

 other continues to increase in size, its basal segments attaining the same diameter 

 as the brachial upon which it rests. The atrophied branch from the original axillary 

 stage of the growing brachial serves as the base from which there extends outward 

 a long tentacular structure with no phylogenetic history, which forms within itself 

 a series of skeletal braces as necessity requires, and which is in every way exactly 

 comparable to a cirrus, which also is a long tentacular structure with no phylo- 

 genetic history, forming within itself a series of skeletal braces as necessity requires, 

 excepting only that it bears ambulacral structures along its ventral surface." 



The quoted statement, apparently, rests on the assertion of Carpenter 

 that the pinnules arise by a bifurcation of the arm, which has been shown 

 to be erroneous (see page 78-79). Further, the pinnule does not at all "ab- 

 ruptly cease its development" after the formation of the two first ossicles. 

 There is a continued augmentation of the number of pinnule joints till 

 they are all formed (comp. text-figure 10), and not the slightest difference 

 is observable between the two proximal and the following joints in regard 

 to shape and development in the growing pinnule; there is no foundation 

 at all for holding the two proximal pinnule points entirely different from the 

 rest of the pinnule joints in morphological value. 



^ This statement regarding the place of formation of the young stem-joints b also in contradiction 

 with Carpenter's own observations: "New segments are being developed in the interval between the highest 

 of these (the stem-joints) and the base of the calyx," he says correctly (page 730). 



"Joh. Walther. Untersuchungen iiber den Bau der Crinoiden. Paleontographica, xxxii. 1886. 



