174 F. R. C. Reed—On the genus Trinucleus. 
ridges in Zrinucleus are homologous with those in other genera, such 
as members of the Olenide. But if we maintain that the marginal 
suture represents the facial sutures, great difficulties seem to be 
introduced. For the relics or rudiments of the compound eyes, 
according to Beecher’s theory, would then have to be sought at the 
edge or below the edge of the fringe, and the ocular ridges as they 
now exist must have been shortened considerably. The ocelli then 
would not be placed at the original terminations of these ‘‘ eye-lines”’ 
but somewhere along their course, and could not be homologous in 
position or origin with the compound eyes. The larval characters of 
T. concentricus, which has the ocular ridge and tubercle developed, do 
not lend any support to the view that the ocular ridges have been 
reduced in length and are shrunken remnants of longer ridges. Thus 
we see another line of evidence tending to contradict Beecher’s theory. 
4, Eyes. 
The absence of eyes in genera of the higher families is found to be 
accompanied by modifications in the head-shield on the same lines 
(apart from the development of a fringe), and no genetic deductions 
can be drawn from this want of visual organs. In the Illenide, 
Phacopide, and Cheiruride we find instances of their absence. 
On the other hand, the presence of compound eyes is not invariably 
associated with the retention of facial sutures, as we find in the case 
of certain species of Acidaspis, in which the sutures have been 
obliterated or lost by the coalescence of the free and fixed cheeks. 
The extension of the ocular ridge to the postero-lateral angle of 
the genal area may find its explanation in the continuation of the 
same structure on the ‘‘ocular segment’? in larval forms and some 
adults of the Mesonacide, and if this is a correct view it would mark 
the original course of the posterior branch of the facialsuture. There 
is, however, another explanation possible by which we could regard it 
as merely the,result of the concentration of those nervures which we 
found in some of the early species of Zrinucleus convergently trending 
to this angle. The enlargement of one nerve at the expense of the 
others in this outer portion of the genal area may have taken place 
contemporaneously with the development of the ocular ridge from the 
axial furrow to the ocellus. And in support of this view we find 
traces of a similar strong nervure in Ampyx nudus, which has a 
facial suture placed much further out and independent of it. 
It may be here mentioned that no lenses have yet been detected in 
the so-called ocelli of any species of Zirinucleus, though they are well 
developed in Harpes and are schizochroal. The visual function of 
these genal tubercles in Zrinucleus is generally assumed, but it may 
be that they had some other sensory function, or that their visual 
powers (if they are regarded as degenerate compound eyes) have 
become obsolete. The tubercle on the pseudo-frontal lobe of the 
glabella, which is generally present in Zrinucleus, and has precisely 
the same appearance as those on the cheeks, may be representative of 
the ‘dorsal organ”’ of the Phyllopods, which is supposed to be excretory 
rather than sensory in its function. But it may represent a median 
unpaired ocellus, and it is frequently present in species which have 
si 
Ee 
