Growth of Arachnoides placenta — Hines and Kenny 
231 
Fig. 1. Selected examples of A. placenta. 
The period of growth occurs during the part 
of the year when sea water temperatures are 
rising (Fig. 3). 
The relationship between diameter and 
weight of test for 125 selected specimens is 
shown in Figure 4. The equation for the rela- 
tionship is of the form 
log W = 3.061 (log d) — 1.519 
where W is the weight of the dried test and d 
the diameter of the animal. 
DISCUSSION 
Extrapolation from the known data would 
suggest that metamorphosis occurs during Sep- 
tember. If it is assumed that the length of larval 
life is similar to that for another tropical 
echinoid, Tripneustes esculentus, which is two 
months (Lewis, 1958), it may be deduced that 
Arachnoides placenta spawns in June or July. 
Lewis (1958) lists July and August as the 
spawning period for T. esculentus ; and Hyman 
(1955), quoting Mortenson, notes that DAellita 
sexiesperforata spawns in March and April in 
the West Indies. It would appear that the 
West Indian species spawn during the early 
summer for their locality, while A. placenta 
spawns during the Queensland winter at sea 
temperatures between 20 °C and 22 °C. 
The occurrence of some Arachnoides in- 
dividuals of diameters up to 36 mm suggests a 
life span of up to five years, assuming the rate 
of growth for older animals to be similar to 
that calculated for specimens up to the third 
year. Crozier (1920) suggested four to five 
years as the normal life span for the similar 
but larger Atlantic species Mellita sexiesper- 
forata at Bermuda, and Moore (1934) esti- 
mated four to eight years as the duration of 
life for Echinus esculentus in Britain. Lewis 
(1958), working with Tripneustes at Barbados, 
was not able to determine a life span because of 
commercial fishing for the species. 
The recording of a specimen of A. placenta 
of 96 mm diameter from Lindeman Island 
(Clark, 1946) suggests the possibility of a 
situation similar to that described by Moore 
