2. THE SPECIES OF KRILL
23
FIG. 6. The petasmae of different species of the genus Thysanopoda. a, T . monacantha;
b, T . cristata ; c, T . tricuspidata ; d, T . aequalis ; 0, T . subaequalis ; f, T . obtusifrons ;
g, T . pectinata; h, T . orientalis; i, T . microphthalma; j, T . acuti;frons; k, T . cornuta;
1, T . eqreqia; m, T . spinicaudata; (a-d, f-h, j-1, after Boden et al., 1955; e, after
Boden, 1954; i, Mauchline, unpublished; m, after Brinton, 1962a).
vary within a species although not, as far as is known, to the degree
shown by this species. Usually, the general form remains relatively
constant but small variations in the relative dimensions of the processes
may be present ; for example, this can be seen by comparing Einarsson’s
drawing (Fig. 7e) with that of Boden et al. (Fig Sj) of the petasma of
Thysanopoda acutifrons Holt and Tattersall.
Members of certain pairs of species are very difficult to distinguish
from one another. Thysanopoda aequalis Hansen and T . subaequalis
Boden were first separated by Boden (1954) and reference should be
A.K.B.-7
2