OUTLilSrE OF THE YOUNG STARFISH. 61 



series, the relations of -which it is impossible to determine. In this con- 

 nection I wonld 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 de- 

 velopment ; this I presume to be somewhere about fourteen years.* 

 They begin to spawn before that time, as sjieciraens have been success- 

 fully fecundated which evidently 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 ex- 

 tremity 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 (4) (the ovarian plates ?). The terminal plate (4) is perfectly well 

 defined at the extremity of each ray, and, by cutting out the remainder 

 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 out- 

 line. 



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 appearance of a small Oreaster. The 

 median line of spines is supported by a long, narrow limestone plate, ex- 

 tending distinctly from the basal plate almost to the terminal radial, 

 plates totally independent, also, of the prolongation of the ovarian plates 



* For an account of the method adopted by Professor Agassiz for ascertaining the age of many 

 of our marine animals, see Proceed. Esse.x Inst., 1863, p. 252. 



8 



