56 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



VI. THAUMATOMETRA NUTRIX Mortensen. 



(Plate XXVIII.) 



The peculiar viviparous habit of this Uttle Crinoid, carrying the Pen- 

 tacrinoids on its pinnules, was described in detail in my report on the 

 Crinoidea of the Swedish South Polar Expedition/* to which reference must 

 be made. 



As the material consisted of only one very poorly preserved specimen, 

 with the arms broken and only a few pinnules left, it could hardly be expected 

 that all the developmental stages should be represented; it is rather more 

 surprising that the Pentacrinoids should present a fairly complete series from 

 the youngest stage to the stage when they are ready to detach themselves. 

 A few eggs were found in the marsupia, but none of them was fertilized, 

 and no information can be given of the cleavage and the larval development. 

 Only the growth changes of the Pentacrinoids can be described. 



The youngest Pentacrinoid is represented in plate xxviii, figure 1. The 

 vestibulum is not yet opened and the oral and basal plates are still in a rather 

 embryonic condition; their middle part retains the original character of a 

 branching spicule; this feature, however, remains distinct, especially in the 

 oral plates, till a much later stage. There is a distinct naked space left 

 between each two adjoining pairs of basals and orals, and it appears that these 

 plates never join completely in the radial midline. Infrabasalia can not 

 be discerned and apparently do not exist in this species. No trace of the 

 radials or the anal plate is seen. The stalk consists of 11 joints; the middle 

 plate of all the joints is still very conspicuous, the joints being still in a very 

 young stage of development. There is no terminal stem-plate. It is true the 

 figure gives the impression that there is a small terminal plate, but it is not 

 really so; it is only the lowermost joint that has, on account of pressure in 

 the preparation, assumed such a position that it is seen directly from above. 

 This specimen has attached itself to the upper edge of the marsupial wall, 

 not to a joint of the pinnules, as is the case with the other Pentacrinoids. 

 (See plate v, figure 3, of the Report on the Crinoidea of the Swedish South 

 Polar Expedition.) 



The next stage represented is that figured in plate xxviii, figure 2. The 

 vestibulum has opened and the primary tentacles are protruding. The orals 

 and basals have grown considerably, the gro\\'th-zones being indicated by 

 the regular arrangement of the holes. The orals have begun to assume the 

 usual form, concave in the upper part, with the edges outturned. The 

 radial plates have been formed; they are peculiar in having an unfenestrated 

 central part. The stalk-joints have growTi somewhat and are now very hard 

 to distinguish from one another, so that their exact number can not be stated. 



"Wissensch. Ergebn. d. Schwedischen Sudpolar-Expedition, 1901-3, Bd. vi, 1918, p. 17. 



