A MONOGRAPH OF THE EXISTING CRINOIDS 



47 



appear at about the eighth brachial. Proximal to this they are suppressed completely, 

 evidently because, even if they had been present, they would have been far from reach- 

 ing up to the surface of the disk and therefore would have been of no use for obtaining 

 nourishment here. It seemed to Gislen that if the pinnule gap were attributable to 

 historical factors something similar should be found in the fossil ancestors of the com- 

 atulids. 



In all the smallest specimens of recent pentacrinites known— species of Metacrinus 

 and Neocrinus decorus—&\\ the pinnules are developed, and the proximal pinnules are 

 the largest. There is no gap in the pinnulation. 



G^len said it was not until he turned to the fossil species of Pentacrinus— the ge- 

 nus that, according to him, most closely approaches the comatulids— that he found 

 the solution of the problem. In Pentacrinus the ventral perisome rises to a height of 

 half the length of the arms, to the fifth or sixth arm division, or to at least the 

 fortieth brachial if all the series are reckoned consecutively. 



In Heliometra glacialis, which has an unusually voluminous disk, this reaches, 

 when the arms are folded and seen in profile, up to the fifteenth to the eighteenth bra- 

 chial, and the anal tube in one case was observed to reach to the twenty-first brachial ; 

 when the arms are extended the disk reaches only to the eighth brachial, or to the 

 height at which it is joined to the arms. In Notocrinus virilis the disk with the arms 

 folded reaches to the eighth or ninth brachial, and joins the arms at the fifth or sixth 

 brachial. In full-grown individuals of Hathrometra tenella var. sarsii the disk is at- 

 tached at the third or fourth brachial, and when the arms are folded to the fifth to sev- 

 enth brachials; in this species the anal tube is very long and reaches to the twelfth bra- 

 chial. In the pentacrinoids of this species the disk reaches about as high as it does 

 in full-grown individuals, or to the sixth brachial, the anal tube reaching to the tenth 

 brachial, though the brachials are relatively longer in the young than in the fully grown. 

 In still smaller pentacrinoids in which the budding of the pinnules in the distal part 

 of the arms has only recently begim the disk reaches only to the first brachial, and 

 the anal cone to the fourth; but in these pentacrinoids the reduction of the basals 

 and radials has not as yet set in to any appreciable degree. 



According to Gislen the difference in the height of the disk with the arms extended 

 or brought together is hardly noticeable in forms having thin disks. For instance in 

 Heterometra crenulata the disk is attached as far as the sixth or seventh brachial but 

 does not reach noticeably higher when the arms are folded. In Asterometra anthus 

 the disk reaches to the third brachial. 



If the proximal pinnules are very long and slender they may, in spite of a fairly 

 high disk, be of use in taking up nourishment or in functioning as tactile organs about 

 the mouth, as in Heliometra glacialis. With an enlarged disk, however, the point 

 may be reached where the proximal pinnules cannot reach to the ventral side of the 

 disk; they are then excluded from playing any part in conveying nourishment, or 

 from functioning as tactile oral pinnules. If the perisome then bulged out around 

 them and they became imbedded in it, evidently their reduction and disappearance 

 would not be far off. 



Gislen noted that this has often been the case in a number of species of Pt nta- 

 crinus, and that the subangularis group of species shows evident reduction of the 

 proximal pinnules. Chiefly on the basis of characters connected with the stem, but 



