8 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



that to the right bearing externally a normal undivided arm and internally a IIIBr 11 

 (3+4) series, on which the outer arm has the proximal syzygies between brachials 4 + 5, 

 9+10, 13 + 14, and 17 + 18, and the inner has them between brachials 2 + 3, 7+8, and 

 11 + 12. 



Gislen said that in the first two cases it is an inner pinnule that has developed 

 into an arm. The first example is perhaps the most remarkable, presenting a picture 

 of the repetition, so far as possible normal, in more distant parts of the arm of the 

 proximal arm ramification and its distribution of nonmuscular articulations. 



Gislen noted that Mortensen (1920) has described a case of P 3 developing into an 

 arm in Antedon petasus, and that the author has described and figured an arm of 

 Thaumatocrinus sp. forked twice in its distal portion. The proximal fork, he said, 

 seems to be due to a mere splitting of the growing point, but the distal branch seems 

 to have been caused by an hypertrophied pinnule. 



Gislen said we have already seen how the new arms in the genus Metacrinus 

 develop in normal cases on the inner side of a main arm, and asks: What, then, is 

 the condition at the first ramification, the IBr axillary? Here there can be no question 

 of the new arm developing on the inner side of an arm branch. A comparison of the 

 material that has been considered shows that in a great majority of cases it is the right 

 hand pinnule that has been strengthened and has become equivalent to the mam 

 arm, corresponding to a IBr 7(1+2, 4+5) series hi M. interruptus. In 9 specimens of 

 M. nobilis tennis the right pinnule is strengthened into an arm in 24 cases, the left 

 pinnule in 1 1. In 7 specimens of M. rotundus 25 right and 1 1 left pinnules are strength- 

 ened into arms. In 19 specimens of M. interruptus 73 right and 15 left pinnules are 

 developed into arms. In all 122 right pinnules as against 37 left pinnules have been 

 strengthened. Gislen said that the reason for this may be ascertained. In by far 

 the greater number of cases the first pinnule is on the right side of the second brachial 

 in other words it is the right branch that is suppressed at the first ramification. When 

 the first real arm ramification at last takes place, it is on the right side that the sup- 

 pressed impulse to arm formation is stronger, and therefore it is oftener a right than a 

 left pinnule that develops into an arm. In 9 specimens of M. nobilis tenuis PI is 

 on the right side of the second brachial in 24 cases out of 35, in 7 specimens of M. 

 rotundus in 26 cases out of 30, and in 21 specimens of M. interruptus in 85 out of 100 

 in all, in 135 cases out of 165. Thus it is the right arm that is suppressed in the genus 

 Metacrinus. 



In the genera of the family Hyocrinidae, according to Gislen, Calamocrinus has its 

 first arm ramification after the first pinnule, while the other genera have simple arms. 

 Examples of the arm base hi Calamocrinus up to and including the first axillary are: 

 IBr 10(1 + 2, 5 + 6, 7 + 8), with pinnules on brachials 4 (left), 6, 8, 9; and IBr 10 

 (1+2, 5 + 6, 8 + 9), with pinnules on brachials 4 (left), 6, 7, 9. The first pair of pin- 

 nules is thus completely suppressed, and the first pinnule develops hi the great 

 majority of cases to the left of the fourth brachial the suppressed first pinnule would 

 also have appeared on the left (of the second brachial) if it had developed. The first 

 arm ramification is also usually caused by the strengthening of a left pinnule. Thus 

 here also there is a connection between the development of the first pinnule and of the 

 first arm ramification similar to that which may be made out hi the genus Metacrinus. 



