518 PYCNOGONIDA CHAP. 
interwoven in a protoplasmic syncytium, whose middle parts are 
occupied by the nuclei and whose inwardly-directed ends form 
the retinal rods or bacilli. The pigment-cells of the inner layer 
are of various forms, those towards the middle of the eye being 
small and flattened, those at the sides being, for the most part, 
long and attenuated, so seeming, as Morgan remarks, to ap- 
proximate in character to the retinal elements. The pigment 
layer is easily dispersed and reveals beneath it a median vertical 
raphe, caused by the convergence of the cells of the middle layer 
from either side, and along the line of this raphe the optic nerve 
joins the eye, though its subsequent course to its connection 
with the retinal elements is obscure. It is at least clear that 
the retina is an “inverted” retina, with the nerve-connected 
bases of its cells lying outwards and their bacillar extremities 
directed inwards. 
In a longitudinal vertical section of the eye of a larva 
(Tanystylum), at a stage when three pairs of walking Jegs are 
present, Morgan shows us the pigment-layer apparently con- 
tinuous with the hypodermis just below the eye, and in close 
connection with the middle layer at the upper part of the eye. 
From this we are permitted to infer a development by invagina- 
tion, in which the long invaginated sae is bent and pushed 
upwards till it comes into secondary contact with the hypoderm, 
so giving us the three layers of the developed eye. This manner 
of formation is precisely akin to that described by Parker, Patten, 
Locy, and others for the median eyes of Scorpions and of Spiders, 
and the organ is structurally comparable to the Nauplius- or 
median eye of Crustacea. But neither in these cases nor in 
that of the Pycnogon is the whole process clear, in consequence 
chiefly of the obscurity that attends the course of the optic 
nerve in both embryo and adult. For various discussions and 
accounts, frequently contradictory, of these phenomena, the reader 
is referred to the authors quoted, or to Korschelt and Heider’s 
judicious summary.* 
There seems to be a small structure, of some sort or other, 
between the ocelli on either side. Dohrn thought it might be 
auditory, Loman that it might be secretory, but its use is 
unknown. 
Integument.— The chitinised integument is perforated by 
1 Vergl. Entwickl. d. wirbellosen Tiere, Jena, 1893, p. 664. 
