Schizaster compactus from Western Australia 
321 
Figure 6 Plots of. A, peristome width, B, peristome length, C, periproct width and, D, periproct length, expressed ; 
percentages of test length against test length for Schizaster (Schizaster) compactus. 
gonopores and test size, the same proportion of 
large and small specimens having only two genital 
pores as have four gonopores. Of those that have 
three gonopores, the odd anterior pore may be 
either on the right side or the left side (that is, 
either gonopore 2 or 3). The anterior pair of pore 
pairs are consistently smaller than the posterior 
pair (Figure 6), a feature of the species noted by 
Mortensen (1951). They may be as large as about 
one third the diameter of the posterior gonopores, 
or as little as one tenth. The posterior pair open 
before the anterior pair and expand in area at a 
greater rate. The onset of opening of the anterior 
pair would seem to vary between individuals. 
From a phylogenetic viewpoint these variations 
to the timing of opening of the anterior gonopores 
can be considered in a heterochronic context. The 
plesiomorphic condition within schizasterids is the 
possession of four open gonopores. In those 
individuals that produce only three, the delayed 
opening of the fourth gonopore occurs by 
postdisplacement, that is a delay in the time of 
opening of the gonopore (on the assumption that 
test size is providing a reasonably valid metric for 
time). In those individuals that produce only two 
gonopores, there is also postdisplacement of the 
other gonopore. The three states therefore lie along 
a paedomorphocline (sensu McNamara 1982, 1990) 
10 15 20 25 30 35 40 
Test Length (in mm) 
Figure 7 Histogram showing number of specimens in 
each 1 mm size class in the northwestern 
Australian population of Schizaster 
(Schizaster) compactus. 
