CHAPTER 5 



Description 



a. Size and Shape, Plate I 



The shells (without spines) of Arbacia punctulata, collected at random 

 at Woods Hole during many summers measure 0.6-5.6 cm. in dia- 

 meter and 0.3-2.7 cm. in height. They are roughly twice as broad 

 as high. The spines measure 0.35-2,5 cm. in length. The overall dia- 

 meter (including spines) is 1.3-10 cm. The average animal has a dia- 

 meter of about 4 cm. without spines and about 8 cm. with spines, and 

 a height of about 2 cm. The spines vary in length and in shape in 

 different regions of the body. The spines on the oral surface are flat- 

 tened and short, those on the aboral surface are long and pointed. 

 Jackson (1927) measured 14,100 specimens of Arbacia at Woods Hole 

 during the summers of 191 3, 19 14, and 191 5 and his figures are quite 

 similar to mine; his smallest one measured 0.5 cm. in diameter (with- 

 out spines), and the largest ones 5.3 cm. The most numerous were 

 3-4 cm. in diameter. 



There is considerable variation in the shape of the shell, some shells 

 being more flattened than others, and some quite conical. D'Arcy 

 Thompson (1942, p. 944) in his Growth and Form has an interesting 

 discussion on the shape of sea urchin shells, suggesting that they are 

 similar to drops of liquid and flatten by their own weight, the small 

 ones remaining spherical. The conical shells, he thinks, may be due 

 to accumulation of lighter substances, such as oil in the eggs, in the 

 upper part of the shell. Lowndes (1944 b) has pointed out some of the 

 fallacies in these arguments; he found that the shells will withstand 

 heavy weights (4 kilos) placed on them, without injury, and calls 

 attention to the fact that there is very little oil in the eggs. Measure- 

 ments have been made of the diameter and height of a group of 26 

 dried shells of Arbacia of varying sizes and it was found that the ratio 

 diameter/height in the largest shells (4.2 to 4.7 cm. diameter) was 

 1. 96/1; in the medium sized shells (2.0 to 2.7 cm.) it was i. 956/1; in 

 the smallest shells (1.53 to 1.95 cm.) it was 2.01/ 1. There is no flattening 

 with age. Even in a newly metamorphosed animal, the body is not 

 spherical, but somewhat flattened. 



