NO. 6 ECHINOID DISTRIBUTION AND HABITS — KIER, GRANT 



41 



(fig. 9) of this relative length of the petal to the length of the test 

 illustrates that in specimens up to 65 mm. long the rate of increase in 

 petal length is much greater than in specimens longer than 65 mm. 

 The rate of introduction of new pore-pairs in the petals decreases 

 correspondingly during growth (fig. 10). Between the lengths of 22 

 mm. and approximately 100 mm. 8-10 new pore-pairs are added for 

 each doubling of the length of the test. For example, a specimen 44 



1.0 



UJ 



H.80 



u. 



o 



X.60 



h- 



o 



UJ 



.40- 



.20- 



00 



— 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — 1 — i — 1 — 

 20 40 60 80 100 120 140 

 LENGTH OF TEST 



160 MM. 



Fig. 9. — Meoma ventricosa (Lamarck). Scatter diagram showing propor- 

 tionate increase in length of petals with increase in size of test. This 

 trend is illustrated in a series of photographs on plate 9. 



mm. long has 10 more pore-pairs in a single poriferous zone than a 

 specimen only 22 mm. long. In specimens over 100 mm. long, produc- 

 tion of new pore-pairs has almost ceased, with approximately 41 

 pore-pairs in each poriferous zone of each of the four petals. 



The shape of the anterior petals also changes. In the smallest speci- 

 men (pi. 9, fig. 1) these petals are straight, but in the larger specimen 

 (pi. 9, fig. 4) their ends curve anteriorly. The posterior petals curve 

 slightly posteriorly in the smaller specimens but slightly anteriorly in 

 the larger. Furthermore, the anterior petals are more divergent in 

 the smaller specimen where they subtend an angle of 162 degrees than 



