A MONOGRAPH OF THE EXISTING CRINOIDS 309 



In accordance with the fact that fixation may take place at very different ages, 

 some larvae attaching themselves at the age of two or three days and others not until 

 they are seven or eight days old, the processes that accompany the transformation 

 from the free swimming larva to the young pentacrinoid may pass at very different 

 speeds, and thus it is not possible to say at what age one or another stage in the later 

 development occurs. It may be said only that it is the fixation that hastens the 

 development, while up to that time the developmental processes are going on very 

 slowly, or not at all. 



Mortensen said it often happened that embryos that had attached themselves 

 to unsuitable objects, especially the leaves of Posidonia, dropped off and fell to the 

 bottom of the jar. The reason for this evidently was that diatoms and other micro- 

 organisms growing on the Posidonia leaves hindered the fixation to the leaf. Doubt- 

 less the suctorial disk secretes some kind of fluid by means of which the fixation takes 

 place, but in this case it would not act because the disk could not touch the surface 

 of the leaf itself. 



The embryos lying on the bottom did not die, neither did the developmental 

 processes cease ; but the development proceeded abnormally, resulting in the embryos 

 assuming a peculiar pipelike shape. It is the vestibulum that is first affected by 

 the failure of the fixation. The process of closing does not continue to the end. The 

 fold of the epidermis which normally gradually covers up the invagination stops 

 growing when it has reached about midway. The aboral (previously anterior) end 

 of the invagination remains unaltered. When the vestibulum closes normally a 

 remarkable rotation takes place, the vestibulum wandering from the ventral side to 

 the oral (previously posterior) end of the larva, carrying with it the hydrocoel and 

 other internal structures. While at first the vestibulum is contiguous with the disk 

 of fixation, it now lies at the opposite end of the larva. There has thus taken place 

 an enormous prolongation of the part of the epidermis lying between the disk of 

 fixation and the vestibulary invagination that is, the part occupied by the first (in 

 Antedon second) vibratile band. The closure of the vestibulum is, so to speak, the 

 sign for the commencement of this prolongation. But when the closure is not com- 

 plete the prolongation of this part of the epidermis does not take place there is no 

 stimulus to start it and the vestibulum continues to be contiguous with the disk. 

 Meantime the developmental processes otherwise proceed normally, and the columnals 

 of the young pentacrinoid especially grow rapidly. But gradually there is no room 

 for the stalk, and it thus has nothing left but to curve hi an arch, and the embryo 

 becomes humpbacked. 



In spite of the incomplete closure of the vestibulum the hydrocoel and other 

 internal structures continue their development normally, and as Mortensen's series 

 of normal embryos in the fixation stage was very limited, he found it necessary to 

 use also these pipe-shaped humpbacked specimens for the study of the development 

 in the stage of transformation from the free swimming larva to the young pentacrinoid. 



The fact that these humpbacked embryos may develop into true pentacrinoids 

 differing from the normal ones only hi having the head bend downward, like a droop- 

 ing flower, is of no small interest. Using the Posidonia leaves for the larvae to 

 attach themselves to has thus resulted in an unintentional experiment to test the 

 significance of that remarkable embryonic structure, the vestibulum. 



