XXI NERVOUS SYSTEM—EYES 517 
plicated. Its upper antimere is supphed from the pre-oral, its 
two lateral antimeres from the first post-oral, ganglion, and each 
of these three nerves divides into two branches, of which the 
inner bears six to eight or more small gangla, which annular 
commissures passing round the pharynx connect one to another. 
Of these ganglia and commissures the anterior are the largest, and 
with these the outer lateral nerve-branches of the proboscis 
merge. The immediate origin of the nerves to the chelophores 
is from the median nerve that springs from the under side of 
the supra-oesophageal ganglion to run forward into the proboscis, 
but it is noteworthy that the chelophores receive twigs also from 
the lateral nerves of the proboscis which arise from the post-oral 
ganglia. 
Eyes.—Eyes are the only organs of special sense known in 
the Pycnogons. The deep-water Pycnogons, in general those 
inhabiting depths below four or five hundred fathoms, have in 
most cases imperfect organs, destitute of lens and of pigment, 
so imperfect in many cases as to be described as wanting. It is 
rare for the eyes to be Jacking in shallow-water species, as they 
are, for instance, in Ascorhynchus minutus, Hoek, dredged by the 
Challenger in 38 fathoms, but, on the other hand, it is no small 
minority of deep-water species that possess them of normal 
character and size, even to depths of about 2000 fathoms, 
In all cases where eyes are present, they are simple or 
“monomeniscous ” eyes, four in number, and are situated in two 
pairs on an “oculiferous tubercle,” sometimes blunt and low, 
sometimes high and pointed, placed on the so-called cephalo- 
thorax, or first, compound, segment of the body. The anterior 
pair are frequently a little larger, sometimes, as in Phoaichilidium 
mollissemum, Hoek, very much larger, than the posterior. The 
minute structure of the eye has been investigated by Dohrn, 
trenacher, Hoek, and Morgan. The following account is drawn 
in the first instance from Morgan’s descriptions.' 
The eye of a Pyenogon (Phoxichilidium) is composed of three 
layers, an outer layer of specialised ectoderm cells (hypodermis) 
that secrete the cuticular lens, a middle layer of visual or 
retinal elements, and an inner layer of pigment-cells. The 
elements of the middle layer consist of much elongated cells, 
whose branching outer ends are connected with nerve-fibrils and 
1 Biol. Stud. Johns Hopkins Univ. v., 1891, p. 49. 
