Chap.II. OUTLINE OF THE YOUNG STAEFISH. 41 



two {Fig. 8), and three {Fig. 10) years old. A considerable number of specimens 

 were picked up in this way, and they could all be arranged into very distinct 

 groups, representing the Starfishes of the present and of two previous seasons. 

 There seemed to be no gradation from one group to another, such as we have 

 among the young Sea-urchins, which, in consequence of their manner of breed- 

 ing during the whole year, form series, the relations of which it is impossible to 

 determine. In this connection I would say, that by arranging the Starfishes 

 found upon our rocks into series according * to their size, we are able to obtain a 

 rough estimate of the number of years required by them to attain their full devel- 

 opment ; this I presume to be somewhere about fourteen years. 1 They begin to 

 spawn before that time, as specimens have been successfully fecundated which evi- 

 dently were not more than six or seven years old. It is during the fourth year 

 that the rate of growth seems to be most rapid. A young Starfish, measuring one 

 and a half inches across the arms, was kept, during five months, alive in Mr. Glen's 

 tank at the Museum, and during that space of time it grew to three inches. 



In the youngest specimens (PI. VIII. Fig. 1), it is easy to see how the young 

 Starfish has changed its outline from a pentagonal cross (PI. VI. Fig. 11) to the 

 one here represented. The original plates are sufficiently distinct to enable us to 

 trace the process. The arm-plates at the extremity, have been pushed away from 

 the body by the addition of new spines formed at the base of the ray, and on 

 each side of the interradial plates (/ a ) (the ovarian plates ?). The terminal plate {l 2 ) 

 is perfectly well defined at the extremity of each ray, and, by cutting' off the re- 

 mainder of the arm, and bringing the extremity of the ray close upon the disk, 

 we should have our former pentagonal Starfish almost identically the same ; the 

 only change being the greater stiffness of the suckers, the more rounded character 

 of the spines, as well as their greater number upon the original radial plates. The 

 spines have almost entirely lost their fan-shaped embryonic type, and are gradually 

 assuming the aspect of the full-grown rounded spines of the adult Starfish. Here 

 and there, however, a spine still occurs which has retained its fan-shaped outline. 



Owing to the elongation of the ray, the single median line of spines stands out 

 very prominently, and this, together with the rows of large spines extending from the 

 interradial plates on each side of the rays, gives to the young Starfish the appear- 

 ance of a small Oreaster. The median line of spines is supported by a long, narrow 

 limestone plate, extending distinctly from the basal plate almost to the terminal 

 radial, plates totally independent, also, of the prolongation of the ovarian plates (jt? ) 

 which make a broad binding on each side of the ray, uniting with the terminal plate 



1 For an account of the method adopted by of our marine animals, see Proceed. Essex Inst., 

 Professor Agassiz for ascertaining the age of many 1863, p. 252. 

 tol. v. 6 



