6 BULLETIN OF THE 



the form of calcareous spicules, t 1 - 1 5 , of the larger U, beginning with 

 one, t\ just south of the inadreporic opening; followed by a second, t", 

 a little east of south of the first ; a third, t 3 , north of east of the second ; 

 a fourth, t*, east of north of the third ; and a fifth, t 5 , about due east 

 of the first. With these alternate the rods of the smaller U, the first, 

 g 1 , being placed about east of the madreporic opening, the second, g 2 , 

 third, g z , fourth, g\ and fifth, g 5 , alternating respectively with the 

 1st -2d, 2d -3d, 3d -4th, 4th -5th, of the larger U. The members 

 of the larger U are the terminals ; those of the smaller U the genitals.* 

 Between the first genital and the fifth terminal lies a broader space than 

 between other consecutive plates, which is the open part of the larger 

 U. It is an unclosed region which forms the brachiolarian notch. As 

 the brachiolaria is slowly absorbed, this notch is more and more reduced 

 in extent, until it is almost wholly lost, when by this reduction the two 

 Us become rings forming the abactinal calcareous surface of the young 

 starfish. 



If now we rotate the brachiolaria on its axis, through a right angle, 

 so that the madreporic body faces the observer, the anal pole being 

 still below, we have the following perspective of the two Us. It 

 will then be seen that the larger and the smaller Us do not lie in one 

 and the same plane, but that the U formed by the terminals is situated 

 on a greater circle than that of the genitals. This fact explains why it 

 is that the figure formed by the line of the latter is smaller than that 

 of the former. It is as if the U of the terminals was placed on the 

 great circle of a hemisphere, while that of the genitals follows a 

 smaller. The difference in size of the two letters (U) is due to the 

 spherical form of the walls of the stomach of the brachiolaria. 



It is somewhat difficult to understand the exact relationship between 

 the dorsal and ventral or abactinal and actinal t surfaces of the young 

 starfishes, and the relation of the plates which form in these two regions. 

 These two surfaces are separated by the stomach of the brachiolaria, 

 and are not at first parallel, but form an acute angle with each other ; 

 and if the plane in which the plates of the abactinal hemisome were 

 continued to meet that of the primitive extensions of the water tubes, 

 they would cut each other at a small angle. A. Agassiz described them 

 as two " warped spirals," and if in early stages lines be drawn, connect- 

 ing the terminal and genital plates, the planes in which they lie will be 



* The term " genital " is used to denote the same plates as " basal " by Sladen. 

 t " Ambulacralen " and " Antiambulacralen Anlagen" of Ludwig (Entwick- 

 lungs-geschichte der Asterina gibbosa, Forbes). 



