112 
skin obscuring the plates must be removed in order to make the 
outline of these plates distinet; this may be done by applying 
hypochlorite of sodium to some part of the arm, the treatment 
being discontinued before the plates are becoming isolated). In 
Fig. 5. 1. Spicules from an ovary of Op7izo/7jy.Ta australis, in a natural group. — 
2, One of these spicules more enlargecl. — 3. Spicules from the bursal wall ot 
O. australis in a natural group. — 4. Spicules from the bursal wall of O. brevi- 
rima, natural group. — 5. Spicules from the skin of white-ringed variety of O. 
brevirima. 
both species there is on the dorsal side of each armjoint a pair 
of large plates, looking like a continuation of the side arm plates, 
which, however, they are not; these plates do not join in the 
dorsal midline, but are separated here in O. australis through a 
continuous mosaik of small polygonal plates (Fig. 4 . 5 ), in O. hrevi- 
rima through only some few (2—3) such small plates, which do 
not form a continuous mosaik. (Fig. 4 . 3 ). Also the shape of the 
ventral plates is different, the outer edges being mueh more pointed 
in australis than in brevirima (Fig. 4 . 1 — 2 ). 
There are, however, still other differences. In O. australis the 
