A MONOGRAPH OF THE EXISTING CRINOIDS 



71 



In a specimen of the almost perfectly endocyclic Comaster novaeguineae with 67 

 fully developed and 9 short regenerating arms averaging 63 mm. in length the weight 

 was 11.6 grams and the volume 8.25 cc. If all the arms and pinnules had possessed 

 functional ambulacral grooves the total length of these grooves would have been 38.1 

 meters. The ratio between the ambulacral grooves and the volume would have been 

 4.6. But the two posterior radii bearing 34 arms show only a rudimentary ambulacral 

 furrow on the arms, while a large number of the pinnules have no ambulacral grooves. 

 The animal's effective ambulacral grooves are therefore probably at most not more 

 than from 25 to 30 meters long, the ratio thus being between 3.0 and 3.6— unusually 

 small for an ordinary comatulid, but unusually large for a comasterid. 



Most of the comasterids are exocyclic. The most advanced types, as Comaster, 

 Vania, and Capillaster, have combs even upon some of the distal pinnules, and are in 

 most cases exocyclic, except various very slender species of Comaster, as C. minimus, 

 C. novaeguineae, and C. sibogae. Among these, too, the combs are most likely of 

 importance for gathering food. The combs appear in the Comaster type upon every 

 other, or every third, pinnule. About half the genital pinnules are provided with 

 combs. As a result of the development of sex products these pinnules are greatly 

 swollen, clumsy, and presumably not very flexible. It is easy to imagine that this 

 somewhat impedes the combs when engaged in passing the collected food to the 

 ambulacral groove, and that instead it is probably easier to pass the food from one 

 comb to another along the arm toward the mouth. As the combs occur on the arms 

 with no ambulacral groove as well as on the others, an explanation of this sort becomes 

 a necessity, if the combs are to be regarded as of any importance in supplying food. 

 In a specimen of Comanthus parvicirra form vaniipinna the most distal combed pinnule 

 on a grooveless arm was 7 mm. long, while the distance to the nearest ambulacral 

 groove was 18 mm. . , 



Gislen said there is an interesting parallel here to the similar method of obtaining 

 food found in the ophiurans. The evolutionary plan, however, is realized in two 

 different ways in the two cases. In the ophiurans it is the tube-feet, the equivalent 

 of the tentacles of the crinoids, that take over the transportation of food to the mouth. 

 The ambulacral groove, which orginally was undoubtedly ciliated, and used for 

 transporting food to the mouth, became unnecessary and was closed by the margins 

 growing together, finally coming to lie in the epineural canal. Ontogenetical evidence 

 of this,°according to Gislen, is found in the ambulacral groove still being open in very 

 young individuals. In the Comasteridae it is the combs— the transformed dorsal 

 hooks on the ends of the pinnules— that have begun to take over the transportation 

 of nourishment. The process of evolution has not gone so far here, but there is an 

 evident tendency toward the elimination of the ambulacral furrow. The process, 

 however, takes place in a way different from that seen in the ophiurans. The epi- 

 thelium of the ambulacral groove degenerates so that the difference between it and 

 the surrounding epithelium disappears. In both cases there results a nonciliated 

 arm. In the ophiurans the ciliary groove becomes enclosed in an epineural canal 

 and is retained as a cord, now exclusively nervous, while in the comasterids the ambu- 

 lacral folds are smoothed out and the ambulacral groove obliterated, the complete 

 disappearance in this case evidently being possible because there are already nervous 

 systems developed in the arms. 



