20 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



Their rapidity of growth is largely dependent upon the 

 abundance of their food supply. Very young starfishes may 

 double their size within a few days when food is abundant. The 

 older ones grow comparatively more slowly, but far more rapidly 

 than was formerly supposed. The rate of growth of our common 

 starfish is discussed on page 40. 



The previous food supply has a great effect on the size of 

 the animal when sexual maturity is reached, some individuals 

 being several times the size of others at the first breeding season. 

 After the discharge of the sexual products growth is resumed, 

 and may continue year after year, the oldest animals being as 

 a rule the largest. 



When no food is to be obtained, however, the starfish can live 

 for months without apparent loss of vitality, the size being some- 

 what decreased in the meantime. 



EXTERNAL STRUCTURE 



The class Asteroidea, which comprises the starfishes, is dis- 

 tinguished from the other classes of Echinoderms by the presence 

 of a central disk which merges gradually, without sharp de- 

 marcation, into the broad, radially arranged arms, or rays. The 

 rays themselves are hollow, and contain portions of the digestive 

 and reproductive systems and other organs. 



The upper surface of disk and rays is termed the aboral, or 

 abactinal surface, while the lower surface, having the mouth in 

 the center of the disk, is the oral, or actinal, surface. 



Between the bases of. two of the rays on the aboral surface is 

 situated a peculiar skeletal plate, the madreporic plate, or madre- 

 porite. In spite of its small size this plate is often conspicuous 

 from the fact that it is of a different color from that of the adja- 

 cent surface of the body. If examined with a lens it may be seen 

 to be pierced by numerous irregular openings, by means of which 

 the sea water from the exterior may enter the water-vascular 

 system (Plates II and III). 



This madreporic plate may indicate the plane of symmetry in 

 the body of the starfish, for a line passing through this plate 

 and the axis of the ray opposite will divide the body into two 

 nearly symmetrical portions, sometimes spoken of as right and 



