THE EYE OF PECTEN. 61 



defined space as a " Hauptader des Augenstieles" to which 

 these lacunas belong (Rawitz [25], p. 105). Neither do they 

 always surround the nerve (Schreiner, p. 11). 



Whilst the long sensory tentacles are, in the living animal, 

 continually in motion, being reti'acted and again extended, 

 and moved from side to side, the eyes are practically motion- 

 less and point fixedly in one direction only. They contract 

 and may move away from a point of stimulation, this being 

 rendered possible by means of muscle-fibres, which lie longi- 

 tudinally arranged, near the epithelium (PI. 6, fig. 1, Mus.). 

 The latter are narrow fibres, and are not striated, as figured 

 by Patten. Striated muscles do occur, though elsewhere, in 

 the mantle-edge of Pecten (45). The muscles occur on all 

 sides of the eye-stalk. They terminate, according to Rawitz, 

 always at the proximal end of the optic vesicle and are never 

 to be found higher ([25], p. 105). Rawitz has presumably 

 taken the finer muscle-fibres, which do extend up to the 

 cornea, for connective-tissue fibres. Schreiner found prac- 

 tically no muscles in small eyes ([30], p. 11), and states that in 

 P. islandicus, where they were exceptionally well developed 

 on the shell side, thev could be traced to the entrance of the 

 distal branch of the optic nerve. I have traced them to this 

 point in P. maximus, but more delicate fibrils (PI. 6, fig. 1, 

 M.f.), staining quite differently from connective-tissue fibres, 

 extend under the epithelium as far as the edge of the cornea, 

 and are, moreover, present between the cornea and the lens 

 (PI. 6, fig. 1, N.Lf.; PL 7, fig. 7, Lf.). These are evidently 

 the " fine smooth fibres " mentioned by Patten in contra- 

 distinction to his "long striated muscle-cells " of the lower 

 part of the eye-stalk. These fibrils do not, however, enter 

 into any connection with the epithelial cells bounding the 

 cornea, and Patten's "ciliaris " does not exist. They will be 

 referred to again when discussing the fibres situated between 

 the lens and cornea. 



Ganglion cells do not occur scattered in the connective 

 tissue of the eye-stalk, a fact already noted by Patten's 

 successors, who criticised his observations on this, as on other 



