A MONOGRAPH OF THE EXISTING CRINOIDS 6 



described by Mortensen for Antedon petasus the main arm always retains its char- 

 acter of chief branch, and the pinnules originate as small buds alternately to right and 

 left. On the posterior arms the only difference is that the growth of the main arm is 

 retarded, so that the pinnules reach their full size while the main arm grows not at 

 all, or only slowly. Therefore these arms do not terminate in an axillary supporting 

 two pinnules, but on the last segment that gives rise to a ramification there is an almost 

 full grown pinnule, and a main arm composed of a few segments. As soon as more 

 than one or two segments have been formed on the main arm, a new pinnule appears. 

 The main arm, therefore, keeps its ramified character and never develops as a pinnule. 



According to Gisle'n the obstructive or inhibitive factor that causes the formation 

 of pinnules instead of arms may in certain abnormal cases develop two pinnules on 

 a brachial instead of one pinnule and one main arm, or two main arms. He con- 

 cluded from this that sometimes the suppressive factor may produce a reversed 

 effect that what ought to have been a main arm has become 8 pinnule, and the 

 reverse. He regards hypertrophied pinnules as the result of removed obstruction. 



Gisle'n classified arm regeneration in the crinoids as follows: 



1. Restorative: Replacing of a single arm broken off; this occurs in all crinoids. 



2. Reproductive or pseudo-augmentative: From a fracture one or more axillaries 

 with two or more arms are reformed; thus the number of arms is the same as before 

 the breakage. Presumably this occurs in all more or less full grown crinoids. It is 

 called pseudo-duplicative when one axillary and two arms are reformed, and pscudo- 

 multiplicatiiie when several axillaries follow each other, and therefore more than two 

 arms are reformed from a single fracture. 



3. Augmentative: From a fracture a greater number of anns are formed than the 

 broken limb possessed. This occurs in comatulids, and presumably also in Diplo- 

 crinus and Teliocrinus. It may be duplicative, when one axillary and two arms are 

 formed from the fracture, or multiplicative, when several axillaries following one 

 another, and therefore more than two arms, are formed. 



4. Reducing: From a fracture fewer arms are regenerated than the lost limb 

 possessed. This is theoretically conceivable, though it is not known with certainty. 



Gisle'n said that the young of the stalked crinoids are known only hi a very small 

 number of examples, and ordinarily they are in such an advanced stage that not much 

 can be determined regarding their method of arm augmentation. 



Augmentative arm regeneration in pentacrinites probably occurs in Teliocrinus 

 and also in Diplocrinus, according to Doderlein. The drawing of a young Neocrinus 

 decorus left by P. H. Carpenter seems to show that the formation of arms from the 

 IIBr axillary takes place in the same simple way as the formation of the two arms 

 on the IBr axillary in the comatulid pentacrinoid the arm tip forks into two 

 processes that are equally favored in further growth and so develop into two new 

 arms. 



Doderlein said of the young of species of Metacrinus from the Valdivia expedition 

 that he examined, "Ich konnte bei den jungen Exemplaren nachweisen, dass sich an 

 einen oder dem anderen Armstrahl noch unmittelbar vor seinem Ende eine Axillare 

 ausgebildet hatte, d. h. eine Gabelung angelegt wurde." In his somewhat earlier 

 work on the genus Metacrinus in the Siboga report he published a figure of an arm 

 ramification in M. acutus in which "die Arme a.uffallend ungleich sind." 



