352 W. F. R. WELDON. 
transversely to the long axis of theembryo. In the thoracico- 
abdominal region this transverse arrangement is particularly 
well seen ; the nuclei in this region forming a series of rings, 
concentrically arranged round a point which occupies the 
apex of the papilla, and which indicates the position of the 
now closed blastopore. 
The last stage figured is a fully-formed Nauplius (see fig. 9) 
which has already undergone an ecdysis. The external ap- 
pearance of this embryo at this stage is greatly modified by 
the growth of the thoracico-abdominal papilla, which has 
greatly increased in size, and has at the same time become 
folded forwards over the cephalic blastoderm. The optic 
lobes are larger than before, and each lobe is divided into an 
outer retinal portion, in which the nuclei are larger, and 
exhibit a more definitely concentric arrangement; and an 
inner, ganglionic region, with smaller and more densely 
crowded nuclei. The optic ganglion is continuous with that 
at the base of the first antenne. The first antenne them- 
selves are much larger than before; the free portion of each 
projects transversely outwards for nearly half its extent, and 
then becomes bent at right angles, so as to project directly 
backwards. The bases of the first two antenne are connected 
by a prominent ridge, which is crowded with nuclei, and which 
overhangs the mouth; so that the mouth itself is not visible 
in the uninjured Nauplius. The second antenne arise behind 
the first, and below (dorsal to) the transverse swelling just 
described ; they are already distinctly biramous, the outer 
branch being the larger. The mandibles are partly concealed 
by the antenne, and are still relatively small ; they are seen in 
section to be distinctly biramous. The thoracico-abdominal 
rudiment is now much swollen at its base, and is prolonged at 
its extremity into a flattened papilla of considerable size, with 
a rounded and very slightly emarginate apex, which is folded 
over the cephalic portion of the embryo. This mode of origin 
of the ventral flexure, by the growth of a papilla which bends 
forwards as it becomes larger, is quite similar to that de- 
scribed by Reichenbach in Astacus, and by Nusbaum in 
