A MONOGRAPH OF THE EXISTING CRINOIDS 63 



Navicula libellus, Nitschia panduriformis var. minor, Synedria affinis, Trachyneis 

 aspera, Thalassiosira gelatinosa, and Thalassiothrix nitschioides. 



Mrs. Cleve-Euler's opinion on the diatoms identified was that the major part of the 

 species represented are of benthonic origin. The species of Cocconeis are epiphytes on 

 p.lgae and similar material. Some of the others, as Adinocydus, Adinoptychus, Coscin- 

 odiscus, Cydotella, Hyalodiscus stelliger, and Paralia, are tychopelagic (=semipelagic), 

 mostly heavy types that keep to deeper water. Thalassiothrix is pelagic. Mrs. 

 Cleve-Euler was unable to find any pronouncedly pelagic genus, such as Chadoceras, 

 in either of the first two samples. She said of the last two, "Possibly setae of true 

 pelagic plankton types, such as Chadoceras, are present, but they could not be de- 

 termined," and added, "It does not seem probable that they [true pelagic forms] are 

 devoured to any extent worth mentioning by these animals." 



Basing his opinion on the evidence afforded by the samples of excrement, and in 

 the light of Mrs. Cleve-Euler's statements, Gislen considered it clearly manifest that 

 the crinoids live upon a mixed diet of detritus, benthonic microorganisms, and plank- 

 ton, but that the detritus plays a very important part, at least for some of them. 



FORMATION OF THE AMBULACRAL GROOVE AND FEEDING IN THE COMASTERIDS 



Gislen noted that the ambulacral groove is very variable in its formation and 

 offers a great deal that is interesting if one follows its varying extension along the 

 arms and pinnules. 



The ambulacral groove originally extended to all the arms and appeared on all 

 the pinnules. This primitive condition is still found in the stalked crinoids, for 

 instance in Metacrinus and Rhizocrinus. In the comatulids, however, a varying num- 

 ber of proximal pinnules have developed into tactile organs and have been trans- 

 formed into the so-called oral pinnules, which have no ambulacral groove or gonad. 

 In most of the Bathymetrinae there is only one pair of these on each arm, Antedon 

 pdasus has two pairs, and A. mediterranea three. Simultaneously with the change of 

 function their appearance also changes, and they generally become long, flagellate, 

 and exceedingly flexible. They are very inconsiderably transformed in the Himerome- 

 tridae, Stephanometridae, Charitometridae, and Thalassometridae, while a number of 

 the Mariametridae, Calometridae, and Colobometridae, and most of the Macroph- 

 reata and Comasteridae show very advanced specialization. 



Gislen said another reason why the extension of the ambulacral grooves differs from 

 the usual type on certain pinnules is found in the development of the gonads. In 

 most of the comatulids the first gonad appears on the first postoral pinnule, and they 

 appear on all the pinnules following until far out on the arms. In the distal pinnules 

 the gonads are not so strongly developed, and they gradually become partly, or com- 

 pletely, obliterated. In certain types, however, the gonads are restricted to certain 

 pinnules, the genital pinnules, and in correlation with their exclusive function as egg 

 or sperm producers, these have their ambulacral groove and their hyrocoele more or 

 less strongly reduced and become stout and clumsy. The distal pinnules, on the other 

 hand, begin to play the role of respiratory organs and to become specialized for food 

 collecting. They often become very much longer and more slender, and have their 

 tentacles and ambulacral furrow very strongly developed. Thus at least certain 

 species of the family Tropiometridae have no ambulacral groove on the first 8-12 pairs 



