24 BULLETIN OF THE 



The ambulacra] s follow the law of formation of the other arm-plates, 

 ■with the exception of the terminals. The first ambulacral to form is 

 nearest the circumoral, and new plates are added aborally to the first 

 formed. The new ambulacrals are protected by the cap-shaped termi- 

 nals. The ambulacrals originate as elongated rods, with axes at right 

 angles to the line of the radius. The two members of a pair do not 

 necessarily arise simultaneously. The position of origin is nearer the 

 centre of the arm, or nearer its median line than the peripherj'. 



In the earliest condition in which the ambulacrals were seen, they 

 had the form of small calcareous nodules, one on each side of tlie 

 median line of the arm (PI. II. fig. .3). In older stages these nodules 

 elongate into bars, growing from the middle line towards the side of the 

 arm on its actinal region (PI. II. fig. 4). By an increase in the length 

 of the ambulacral bars, they bridge the interval between the middle 

 line of the arm and the lateral extensions of the terminals, although 

 they never join the last mentioned structures (PI. III. figs. 1, 2). 



Each ambulacral bar has the following form. Near the middle line it 

 is enlarged, while on the aboral and adoral borders it is concave, in 

 order to leave an interval or space for the passage of the legs to the 

 ampullee. 



As the growth of the arm of the starfish goes on, and new pairs of 

 ambulacrals are formed, the terminals are pushed out more and more 

 from the disk. At the same time the ends of the ambulacrals approach 

 one another on the median actinal line of the arm, and ultimately 

 become articulated together. Before, however, they join, they bifur- 

 cate on the median line, and form an iipper and a lower spur, as in the 

 circumoral calcareous rint;. As in earlv stages of the starfish, there are 

 only two rows of feet, one on each side of the median line ; the rows of 

 openings for the passage of the feet are also in two lines. It is only in 

 young starfishes of considerable size that we find four rows of openings 

 between the ambulacral rafters. In all the specimens figured there are 

 but two rows of feet. 



The voung stages of Asterias studied bv me were never found to have 

 spmes on the ambulacrals, and neither in the oldest nor in the youngest 

 was there any median row of plates or spines of an embiyonic nature on 

 the actinal side of the arm. 



Second Interhracliial. — "When the growing starfish, in which tlie 

 arms ha\e pushed themselves out to a considerable size, is looked at 

 from the actinal side, there will be seen in the interradii, in the space 

 left between the marginals and the abaxial end of the interbrachial ex- 



