PINEAL EYE IN LACBETILIA. 177 



most posterior point (fig. 2) ; the extreme length of the cap- 

 sule is 1*4 mm. Special fibres cross from the capsule wall to 

 the edge of the lens, and, being connected with the tissue 

 immediately surrounding the retina, may serve the purpose of 

 keeping the eye in position, and thus represent the rudiment 

 of a structure of importance when the eye was fully func- 

 tional. The capsule in its hinder part contains much irre- 

 gularly scattered connective tissue with nuclei, its anterior 

 part, however, being free from them. Within the capsule 

 breaks up an artery (figs. 2 and 7, B.v.) whose branches ramify 

 amongst the fibres behind the eye ; this special blood supply 

 is a prominent feature in connection with the organ in all the 

 forms examined. 



The eye lies with its long axis directed upwards and for- 

 wards in the most anterior part of the capsule ; figs. 7 and 8 

 show the relative position of the eye in its capsule with regard 

 to the brain and the parietal foramen. 



Structure of the Eye. — Through the kindness of Prof. 

 Moseley I have been able to examine the structure in a fresh 

 specimen, and, notwithstanding the fact that the organ cannot 

 now be fully functional, the retina is fairly well developed. 



The eye has, roughly speaking, the shape in section (PI. 

 XIV, fig. 2) of a cone, the base of which lies turned towards 

 the surface, whilst the pineal stalk is connected with the apex. 

 The walls of the optic vesicle are divided into two parts, (1) 

 an anterior ; (2) a posterior ; of which the former forms the 

 lens, and the latter the sensitive structures. 



(1) Lens. — The lens of the pineal thus differs markedly 

 from that of the paired eyes, where it originates as a secondary 

 structure by invagination of the epiblast, whilst in the former 

 it is apparently directly the product of the brain wall itself, 

 and equivalent in position to that part of the paired optic 

 vesicles which after invagination forms the retinal elements. 



De Graaf has likened the eye to that of such Invertebrates 

 as Cephalopods and Pteropods ; but, apart from other differ- 

 ences which exist between the two in regard to both develop- 

 ment and structure, the lens is not in the least degree com- 



