MONOGRAPH OF THE EXISTING CKINOIDS. 505 



At this stage the arm is free from the base of the IBr^ later the visceral 

 sack extends to the bifurcation, and the whole of the IBr series becomes included 

 in the cup and disk. 



The azygous (median unpaired) tentacles go no farther than the bifurcation. 

 They remain for some time in the center between the two divisions of the arm, 

 while secondary branches from the radial canal run on in the brachial grooves. 



About the period of the development of the IB^ a forked spicule makes its 

 appearance in one of the interradial spaces between the upper portions of two of 

 the radials. This gradually extends in the usual way until it becomes developed 

 into a superficial rounded cribriform plate. 



Simultaneously with the appearance of this radianal plate a caecal process, 

 like the finger of a glove, rises from one side of the stomach toward the plate. 

 The plate increases in size, becomes inclosed in a little flattened tubercle of sarcode 

 and, maintaining its upright position, passes slightly outward, leaving a space on 

 the edge of the disk between itself and the base of the oral plate immediately 

 within it. Toward this space the csecal intestinal process directs itself. It rises 

 up through it in the form of an elongated tubular closed papilla. The summit 

 of the papilla is finally absorbed, and a patent anal opening is formed. 



The azygous tentacles remain persistent for some time in the angles between 

 the two first brachial segments, and finally become absorbed and disappear. 



Regarding the later development of the arms W. B. Carpenter says that at 

 the beginning of the free stage the first primibrachs begin to show a modification 

 of their originally nearly cylindrical form, being somewhat widened out at their 

 lower end, so as to form two articular surfaces which are not, however, directly 

 opposite those of the radials, but slope away from them, so that, while in close 

 proximity with them on the dorsal face, where they are connected by ligaments, 

 they are separated by a considerable interval on the ventral side, where muscle 

 intervenes an arrangement which gives to the muscle fibers the length required 

 for their efficient contraction. The upper end, however, is simply rounded off, 

 presenting 110 proper articular surface, and it is connected with the base of the 

 second primibrach by ligaments only. Although the dorsal surface of the first 

 primibrach is everywhere convex, its ventral face still shows a deep groove along 

 its median line ; this, however, is separated by a distinct layer of calcareous network 

 from the axial canal beneath it, which opens at its lower end between the articular 

 surfaces and at its upper in the center of its rounded termination. 



The IBr, (axillaries) at the commencement of the free stage show, like the 

 IBr,, only a slight departure from the original simple type, their lower portion 

 being incompletely cylindrical, while the upper is somewhat expanded laterally 

 to form a pair of articular surfaces, which look obliquely toward either side and 

 are separated by a projecting median ridge. The lower extremity is rounded off 

 so as to resemble the upper end of the first primibrach and, like it, shows the 

 orifice of the axial canal in its center, while each of the articular surfaces at its 

 upper end shows a similar perforation which is the orifice of one of the two 

 branches into which the axial canal bifurcates in the interior of the segment for 



