A MONOGRAPH OF THE EXISTING CRINOIDS 5 



age of an arm (in the comatulids autotoiny) when the dormant tendency is released 

 following the inception of regeneration. 



Gisle'n said that on an examination of the genus Metacrinus one is struck by the 

 extent to which the odd numbers are more frequent than the even in the division 

 series that is, if the hypozygals are counted as independent segments. Among the 

 specimens of Metacrinus nobilis tennis examined by him there were in the IIBr to 

 VBr series 233 odd to 26 even; in M. rotundus 217 odd to 70 even; in M. interruptus 

 372 odd to 59 even. Moreover, if the number of syzygies in the different division 

 series are examined it is soon found that the series with an odd number almost always 

 have an odd number of syzygies, while those with an even number have an even 

 number of syzygies. So if syzygial pairs are considered as units, as was done by 

 Carpenter, the division series have an even number of segments. Counted in this 

 way, the specimens of M. nobilis tennis examined had 258 series with an even number 

 of segments and only one with an odd number; M. rotundus had 275 even to 12 odd; 

 and M. interruptus had 422 even to 9 odd a total of 955 even series to 22 odd. 



Sperry made a curve for the variability of the IIBr and IIIBr series in M. rotundus, 

 and he also gave a strong preponderance to the even series. He looked for the reason 

 for this in the fact that pinnules on the inside of the arm are closer together than on the 

 outside. If the number of segments is even there will be a smaller number of pinnules 

 on the inside of the division series than on the outside. According to Gisle'n this 

 observation is quite correct, but it does not give an explanation of the phenomenon. 

 The real explanation presumably is that if there is an even number of segments 

 (syzygial pairs counted as units) the new arm will be formed as a branch on the inside 

 of the arm where it is more protected during its early growth than on the outside. 

 How far-reaching this rule is may be seen from the figures given. It can also be 

 confirmed by other facts. Gisle'n cited some examples of reproductive arm regenera- 

 tion. Measurements of the regenerated arms show that the inner arms, even in the 

 case of reproduction, are at first weaker than the outer (main) arms. This is explained 

 by the rule just given, for the regenerate suggests the form that the actual ontogenetic 

 development took. When, as sometimes happens, a new arm develops on the outer 

 side of the mam arm, the ramification often takes an abnormal form. The base of 

 the new arm bends outward in a wide curve, and the two arms diverge not at equal 

 angles from the main axillary, as is usually the case. 



Sometimes, in spite of an uneven number of components in the series, the new 

 arm still appears on the inner side, owing to abnormal pinnulation. According to 

 Gislen the explanation of an abnormal number in the division series is generally that 

 the main arm has aborted and become a pinnule, while a pinnule has hypertrophied 

 and taken over the function of the main arm in other words a shifting of the sup- 

 pressive factor has taken place. 



Gislen said that so far as can be judged from Agassiz's work on Calamocrinus, in 

 this genus too it is most frequently an inner pinnule that is strengthened into an arm. 



Carpenter's figure of the young of Neocrinus decorus shows that in this genus the 

 new arm is formed by the splitting of the growing point. Thus it cannot be said in 

 this case that a new arm developing on the inner side of the main arm would be more 

 protected than one developing on the outer side. In this case, therefore, the number 

 of even segments (syzygial pairs counted as units) in the IIBr and following series is 



