xxi NERVOUS SYSTEM EYES 5 I / 



plicated. Its upper antimere is supplied 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 ganglia, 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 lacking 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 Phoxichilidium 

 mollissimum, Hoek, very much larger, than the posterior. The 

 minute structure of the eye has been investigated by Dohrn, 

 Grenacher, Hoek, and Morgan. The following account is drawn 

 in the first instance from Morgan's descriptions. 1 



The eye of a Pycnogon (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 L'nii: v., 1891, p. 49. 



