28 



STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



radials, which are still widely separated from one another, have assumed a 

 characteristic heart-shape, while the costals still remain slender. The orals 

 are widely separated from the basals; they have a very characteristic shape, 

 having a deep furrow along the middle line, the sides being gracefullj^ bent 

 outwards, as is also the basal part. The stalk is now composed of 27 joints. 

 The 5 upper joints are very short, but wider than the rest and with prominent 

 middle plate; then foUow 2 equally short but much narrower joints. The 

 eighth joint is shghtly longer, the next about twice so long, and from the tenth 

 they have assumed their final shape, as described for the following stages. 

 Plate XIII, figures 4 and 5, represent the fully formed Pentacrinoid. In 



figure 4 the cirri have just begun to 

 appear ; in figure 5 the first pinnule 

 has been formed. The oralia have, on 

 account of the growing of the disk, 

 shifted their position, so that they lie 

 now entirely on the ventral side of the 

 disk and separate from the calyx proper. 

 The anal cone has developed in the 

 space between the anal plate and the 

 adjoining oral plate, so that there is 

 now a large plate both on the outer and 

 the inner sides of the anal cone (plate 

 XIII, figure 5). The radiaha have en- 

 larged considerably, and join with their 

 lateral edges. The costal has enlarged 

 laterally and the axillary has assumed 

 its characteristic shape, with the two 

 oblique articulating surfaces. The arm- 

 joints are short and broad, somewhat 

 thickened at the ends. The pinnule is attached to the twelfth joint, which 

 means that the first pinnule to develop is the first true arm-pinnule, the oral 

 pinnules developing later. In a newly detached young specimen, with 6 to 7 

 arm-pinnules, the oral pinnules are just about to develop, and it is seen there- 

 from that of these latter the first and the fifth to sixth are the larger and of 

 about the same size; they must, accordingly, develop first and contempora- 

 neously, while the second to fourth are much smaller and must appear after 

 these, and also about contemporaneously. Generally the fourth would seem to 

 be the last of them to develop, but in one arm of the specimen mentioned this 

 is shghtly larger than pinnules 2 and 3. The pinnules 7 and 8 have not yet 

 appeared, which is the more remarkable, as later on these become larger than 

 the other oral pinnules except the first. Although the 4 outer oral pinnules 

 contain genital organs in the grown specimens, they are true oral pinnules, 

 without tentacles. 



Fig. 6. — Arm of a young specimen of Comp- 

 sometra scrrata showing order of appearance of 

 oral pinnules. 



