CRINOID GENUS SCYPHOCRINUS ' 43 



stage the ossicle still shows its axillary character, and the ambulacrum of the 

 pinnule empties into the main ambulacrum within the ossicle. 



In the second stage incorporation of the pinnulate arm ossicles is accom- 

 panied by incorporation of one (Marsipocrinus, fig. 19) or more (Cyphocrinus, 

 fig. 18) pinnule ossicles. Various changes are here necessitated in the position 

 of pinnule ambulacra in reference to their respective arm ossicles, as the upward 

 shifting of the mouth has caused a separation of the ambulacra from the plates 

 with which they were formerly in contact. Thus in Marsipocrinus and Cypho- 

 crinus the ambulacra on the incorporated pinnules of the first and second IIIBr 

 pass directly into the main ambulacra (note position of ambulacral covering 

 plates in Marsipocrinus), without grooving the arm ossicles from which the 

 pinnules are given off. 



The third stage is that illustrated in Glyptocrinus and Scyphocrinus, in 

 which the lowest pinnules are almost completely incorporated in the cup. 



In the first and second stages the openings of the pinnule ambulacra into 

 the cup are clearly defined ; in the third stage, however, the opening is lost, as 

 the ambulacra have probably atrophied, or the pinnules become so merged in 

 the finely plated perisome passing into the tegmen that their openings cannot 

 be discovered. 



From all these facts it is clearly evident that the openings in question in 

 Camerate crinoids represent the pinnules which developed immediately follow- 

 ing the first division of the ray in the young crinoid, and became afterwards 

 more or less incorporated by the growth of the calyx. They are comparable 

 to the proximal pinnules in the Comatulids, which differ in many notable 

 respects from those which succeed them. Mr. Austin H. Clark, to whom I 

 applied for detailed information, has been kind enough to furnish me a note 

 embodying numerous examples of this fact, which with his permission I am 

 here inserting: 



Among the Comatulids the pinnules of the first pair, usually also those of the second, 

 frequently those of the third, and sometimes those of the fourth and fifth, are different 

 in structure and function from those succeeding-. They are invariably sterile and without 

 ambulacral groove or tentacular apparatus, and are so modified that, instead of serving 

 as food collectors like the distal pinnules, or as sexual organs and usually also as food 

 collectors like the middle (genital) pinnules, they function as highly sensitive tactile 

 organs, or as spine-like organs of defense. 



In certain genera, distributed in widely different groups (for example, in Antedon, 

 Florometra, Pontimetra, and Pentametrocrinus) , the oral pinnules, composed of numerous 

 very short segments, are greatly elongated and of extraordinary flexibility, calling to mind 

 the antennae of certain crustaceans. In these genera they appear to serve simply as tactile 

 organs. 



In one family (Comasteridae) the outer segments of these tactile pinnules bear long 

 triangular processes which collectively form a prominent comb-like structure: the function 

 of this is not clearly understood ; it may serve as a cleaning apparatus, or merely to 

 increase the intricacy and sensitiveness of the entanglement over the disk. 



7 



