MUSEUM OF COMPARATIVE ZOOLOGY. 21 



the increase in length of the arms, A second pair of marginals is not 

 formed until after the second pair of iuterambulacrals has appeared. 



The forming marginals bridge the intervals between adjacent inter- 

 ambulacrals. They are larger than the interambulacrals, but not so 

 massive, and form curved plates making the curve in the margin of the 

 arms. Their number is not so constant as compared with that of tlic 

 arabulacrals as with the adambulacrals, and in this respect even in very 

 young stages they recall the marginal plates of the adult starfish. 



Oral Ambidacral Plates. — The plates, or calcareous framework which 

 surrounds the mouth of Asterias, date back to very early stages in the 

 growth of the starfish (PI. I. fig. 3). Rudiments of these structures 

 appear while yet the starfish has a disk-like form, and before the com- 

 plete absorption of the arms of the brachiolaria. 



In the earliest condition in which these plates were seen there were 

 but eleven other plates in the starfish body, and these were all found 

 on the abactinal surface. These eleven plates are, of course, the single 

 dorsocentral, the five terminals, and the fi-e genitals. 



The oral ambulacral plates appear on each side of the primitive ex- 

 tensions from the right water vessel, rw, which later form the five radial 

 water-vascular tubes of the arms. They appear in pairs and are tea in 

 number, a pair to each pair of legs. 



In their earliest stages they are spiculate and elongated, their length 

 running parallel with the walls of the water tube (PI. I. fig. 3). This 

 fact is an important one, for it recalls the condition which we have in 

 the ossicles, or ambulacral plates, of Amphiura. I shall speak of this 

 condition later. The elongated rods or spicules have later (PL II. 

 fig. 1) small lateral branches, and a beginning of a network is to be 

 seen. The two members of the pair never grow together laterally in 

 the position that they are at first placed. 



The condition of the watei tube, when the first pairs of circumoral 

 rods form, is briefly as follows. The tube has not joined about the 

 mouth, as the brachiolaria is not yet fully absorbed. The five median 

 water-vascular tubes, rw, are simple protuberances, without lateral 

 appendages. The pre-divided water system is asymmetrically placed 

 as regards the disk of the future starfish, and the five extensions do 

 not project beyond the stomach of the brachiolaifia. In the next form 

 in which the circumoral calcareous rods appear, we find that the brachi- 

 olaria has been wholly absorbed, and the starfish has assumed a stellate 

 form, brought about by an enlargement and growth of the termi- 

 nals (PL II. fig. 2). While the oral plates were placed with their 



