788 " THETIS " SCIENTIFIC RESULTS. 



young specimens of Ptilometra in the collection as small as the 

 larger specimens of pcedophora, and it is possible, therefore, to 

 show that the two are not even nearly allied forms ; the conical 

 centrodorsal, the long cirri and the crowded prismatic pinnules 

 distinguish the Ptilometras at a glance. Moreover, it should be 

 noted that pentacrinoid larvse were not present on the pinnules 

 of any undoubted Ft. macronema [i.e., mulleri] . . It 



is difficult for me to believe that the smallest Ptilometras, too 

 young to have assumed any of their chiiracteristic generic or 

 specific features, shoul 1 be the only ones which are breeding." 



Answering the last statement first, I some time ago'' showed 

 that it is rarely the adults of any species which bear Pentacrinoid 

 young upon their pinnules or cirri, but usually young ones ; the 

 free swimming young always drift to leeward of the parents 

 before settling down ; at the next breeding season another brood 

 drifts over to the place occupied by the brood of the preceding 

 season and settles down upon it. Thus it is that while young 

 Comatulids often bear Pentacrinoids they are very i-arely found 

 on the fully grown. There are some nice examples of this in the 

 Museum at Cambridge, where the cirri of some young specimens 

 of Oomactinia meridionalis are preserved bearing Pentacrinoids 

 upon them, and I have seen some scores of additional cases of 

 the same thing. 



The Comatulids change from their comparatively undeveloped 

 post- Pentacrinoid condition to the perfect form very quickly ; 

 this is accompanied by a rapid development of Pg and the 

 immediately succeeding pinnules, a great increase in the width 

 of the pinnulars and the brachials which make the pinnules 

 appear much closer together and the brachials shorter, and, in 

 species with long cirri, by a rapid (distal) addition to the cirrus 

 segments, the added segments as they appear becoming pro- 

 gressively more and more developed. In the Thalassometridse 

 and in the Tropiometridse the carination of the brachials and 

 pinnules is never present in the very young, but is rather sud- 

 denly assumed at an early stage ; a graphic recapitulation of this 

 may be observed in any regenerating specimen. 



Dr. Clark has failed to mention a single character by which 

 his pcedophora can be distinguished from the young of Ptiloraetra 

 mulleri as predicated from the facts known in regard to the young 

 of related species and, as we have now been able to study a 

 specimen exactly intermediate between the pcedophora stage and 

 the adult condition, there seems to be no escape from the con- 

 clusion I had arrived at, and communicated to him, when Dr. 



^ Vidensk. Medd. fra den Naturhist. Forening i Kjobenhavn, 1909, p. 

 120. 



