20 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



adjoining radius, not in the middle of the interradius. When it has been 

 formed the young Crinoid is ready to open the vestibulum and the embryonic 

 life proper is ended. 



6. THE DEVELOPMENT OF THE SKELETON; THE PENTACRINOID. 



(Plates IX to X.) 



The first rudiments of the skeleton appear at the age of 24 hours (plate 

 ix, figure 1); here the basalia and oralia are seen lying in two half-circles, 

 which are open ventrally. The plates of the two half-circles do not corre- 

 spond exactly to one another; there is some shifting, as is also the case in 

 Antedon. It is evident that the plates do not all appear at one and 

 the same time. The oralia i and n are slightly larger, and there- 

 fore older, than the other 3 oralia, and the basale v has just 

 appeared as a minute grain, while the other 4 have already begun 

 to form small processes. The terminal stem-plate has appeared 

 and 5 stalk-joints. It is noticeable that the joint nearest the 

 terminal plate is smaller than the next one, showing that it is 

 not the first formed. 

 FIG. 2. In plate ix, figure 2, which also represents a larva of 24 hours, 



Three stages fa e skeleton is somewhat more developed ; the terminal stem-plate 



in the devel- 



opment of and some of the basalia and oralia are distinctly branching; the 

 tints'* ^320 stalk-joints still number only 5; here, however, the one nearest 

 the terminal stem-plate is the largest, so that it would appear to 

 be also the first formed in this case. Then an important new skeletal 

 element has appeared. At the upper (oral) end of the stalk 3 very small 

 calcareous grains are seen, which are the first traces of the infrabasalia. 

 Their definite number can not be stated from this young stage, but the 

 following stages afford the proof that there are only 3 of them. 



A laterstage is shown in plate ix, figure 3, representing a larva 30 hours old. 

 The oralia and basalia, as well as the terminal stem-plate, have considerably 

 enlarged and are now more or less fenestrated plates; their relative position 

 has changed somewhat, so that they are no longer arranged in the form of 

 a horseshoe. The vestibulum could not be made out in this specimen, but 

 judging from the curvature of the stalk the specimen must be drawn in the 

 same position as figure 1. Only two of the infrabasalia are seen in the figure, 

 below one of the basals; the third was probably quite concealed by this 

 plate. The stalk-joints have grown considerably in breadth, assuming the 

 shape of a half moon, their transformation into the ring-shaped joints 

 occurring in the same way as in Antedon. In this stage some very small 

 calcareous grains are seen between some of the normal stalk-joints, especially 

 between the fifth and sixth and between the sixth and seventh joints. This 

 might perhaps represent new joints interpolated between the first formed. 

 However, it is by no means certain that they are really intercalated joints. 



