76 WILLIAM PATTEN. 



IV. The Parietal Eye. 



After the median eye-tubes in fig. 59, p. e., unite, a single 

 tube is formed like that in figs. 24 and 25. The solid distal 

 end (fig. 35j m. e.) soon divides into three lobes; the two 

 outer ones just before hatching become filled with black pig- 

 ment, and form the retinas to the ectoparietal eyes {m. e.); 

 the inner, unpaired, one forms the endoparietal eye (m.e'.). 

 There is no distinct cavity in these lobes, but as they are 

 formed from the walls of an invaginated tube^ and as the two 

 outer ones are homologous with the median eyes of scorpions, 

 where such cavities are present, we must regard them morpho- 

 logically as sac-like diverticula from the end of a tube. Their 

 similarity to the common vesicle of the median eyes of scor- 

 pions and their relation to the cerebral vesicle is easily seen 

 in figs. 41 and 42. During the first larval stage the endo- 

 parietal eye is a solid cylindrical mass of cells with rod-like 

 thickenings in their walls, like those in true retinal cells, and 

 filled with dense white pigment (fig. 63). In sections the 

 crumpled refractive plates on these degenerate retinal cells 

 look like coiled or zigzag fibres something like those in 

 fig. 20. As the animal grows older this mass of aborted retinal 

 cells buries itself deeply in the underlying tissue, away from all 

 connection with the exterior (figo 74). In the adult it usually 

 lies below a conical tubercle, situated a little behind the two 

 lenses to the median eyes. In many old specimens this 

 tubercle is replaced by a clear, transparent spot, which is no 

 doubt the remnant of a lens. As the endoparietal eye 

 must itself have been formed by the fusion of paired retinas, 

 it is evident that the distal end of the parietal eye- 

 tube contains the retinas of four originally distinct 

 eyes. These facts are all the more interesting since Claus 

 has shown that the median eye of Crustacea, which in some 

 cases is composed of three distinct eyes, is formed by invagi- 

 nation just as in scorpions and Limulus, 



The distal portion of the primitive eye-tube is converted 

 bodily into the median eye-nerve. Just before hatching its 



