A MONOGRAPH OP THE EXISTING CRINOIDS 7 



other hand, in Metacrinus and forms of a similar type whole clusters of arms are 

 easily broken off, and so there is a greater percentage of loss of ambulacra! furrow. 



Occasionally in the comatulids also there occurs a direct strengthening of pinnules 

 to an equality with arms. In a case of this kind in Antedon petasus described by 

 Mortensen (1920) in 5 arms out of 10 both P, and P a have developed into more or less 

 complete pinnule-bearing arms, P a being often stouter than P,. In another individual 

 examined by Gislen P] on the left anterior ray had developed into an arm 75 mm. 

 long that was as stout as the other arms. From the sixth brachial, where the pinnules 

 begin on the hypertrophied P u there are stout segments and oblique articulations, as 

 on the other arms. Syzygies of normal appearance and distribution also occur, the 

 distal intersyzygial interval being 3 muscular articulations. Gislen noted that a 

 similar case, possibly normal, is found in the so-called Comatula etheridgei. If C. 

 etheridgei (see Part 3, p. 309) is the young of C. rotalaria there is here direct arm 

 development like that in Metacrinus. 



In a specimen of Cyllometra pulchella ( = manca) from Mortensen's collection 

 (station 10) the right posterior radius has a normal undivided arm on the right; on the 

 left there is a IIBr 3 series, the external arm from this being normal with a synarthry 

 between brachials 1 and 2, syzygies between brachials 3+4 and 15 + 16, and the first 

 pinnule on the second brachial, and the internal arm having syzygies between brachials 

 1+2 and 16 + 17, and the first pinnule on the fourth brachial. Gislen recalled that 

 Springer described a specimen of Uintacrinus socialis with 11 arms, as the result of the 

 development of a P a into an arm. 



Gislen said that occasionally, for one reason or another, pinnules develop directly 

 into arms in more distal portions of the arms also. He cited a specimen of Asterometra 

 anthus in which on the right arm of one of the postradial series the seventh brachial is 

 axillary, the pinnule having developed into an arm with syzygies between brachials 

 4 + 5, 12 + 13, 19 + 20, and 27 + 28, a pinnule on each side of the third brachial, butthe 

 pinnules following normally distributed. At the point where the arm divides an 

 entoparasitic gastropod is encysted, and Gislen suggested that it was possibly the 

 irritation caused by the parasite that stimulated the pinnule to develop into an arm, 

 though he noted that many other specimens of the same species had encysted gastro- 

 pods without arm division having taken place. For some reason or other there are no 

 soft parts on the eighth-twelfth brachials, though the distal part of the original arm 

 still has an ambulacra! furrow. He said that this is the most probable cause of the 

 hypertrophy of P 3 on the original arm. The flow of body fluids to the distal parts of 

 the main arm has been cut off, and P 3 has received the surplus and so has developed 

 into a complete arm. It is to be noted, however, that no such abnormal flow of body 

 fluids can be adduced in the three cases following. 



In a specimen of Comanthus pinguis from Mortensen's station 10 one postradial 

 series has two IIBr 4(3+4) series; the inner arms from each of these are undivided. 

 One of the outer arms has a IIIBr 4(3 + 4) series, the other a IIIBr 8(3+4, 5 + 6, 7 + 8) 

 series. 



In a specimen of Cyllometra pulchella ( = manca) from Mortensen's station 10 one 

 ray bears an undivided arm on the right and a IIBr 8(3+4) series on the left. 



In a specimen of Neometra multicolor from Mortensen's station 24 one postradial 

 series has two IIBr 2 series, that to the left bearing two normal undivided arms and 



