5 I 8^ PYCNOGONIDA 



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 legs 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 sac 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 Korsehelt 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 



' Verrjl. Entioickl. d. wirhellosen Tiere, Jena, 1893, p. 664. 



