Explanation of Piate 26. 



Figures 2, 3, 5, 8, 9, 16 are all drawn to the same scale to give their relative sizes. 



Figures 11, 12, 13 are copied from Fraisse and figure 15 enlarged twice 



from a figure by Hensen. The colored figures stained in borax Carmine. 

 Fig. I . Radial section (perpendicular to mantle-edge) of the edge of the mantle 



of Ostrea cirginica: a external , b middle, and c internal fold of mantle'a 



edge; h.e. epithelium of the external wall of the branchial Chamber ; 



B.S. Branchial side of the mantle; b.s. blood Spaces in the mantle; 



cu. cuticle of the shell ; d a branch of the retractor muscle [r.m.] ; e.m. 



extensor muscle; P.N. transverse section of the circumpallial nerve; 



s.e. epithelium, which lies next to the shell. 

 Fig. 2. Five pigmented retina cells from the edge of the mantle of Ostrea virgi- 



nica (for letters see text p. 453}. 



Five similar cells from Mijtilus edidis (for letters see text p. 460). 



Part of section of siphon of Venus mercenaria , showing the pigmented 



epithelium. 



Five pigmented retina cells from the siphon of Venus mercenaria lettera 



as in fig. 2). 



Longitudinal section of the external end of the siphon of 3Ii/a arenaria : 



a internal band of pigmented cells; b space between external and in- 

 ternal row of tentades. 



Magnified view of base of tentacle marked c fig. 6. 



Four pigmented retina cells from the siphon of 3fga arenaria (letters as 



in fig. 2). 



Three similar cells from Solen vagina ,'letters as in fig. 2). 



Epithelium from siphon of Solen ensis (for explanation see text p. 467). 



Transverse section of retina cells of Patella coerulea. The pigment col- 



lected on the walls of the cells (copied from Fraisse, Taf. XXVI fig. 11). 



Three retina cells from the eye of Patella coerulea (copied from Fraisse, 



Taf. XXVI fig. 12). 



Six retina cells from the eye of Haliotis tubercolata (copied from FraisSE, 



Taf. XXVI fig. 13). 



A group of retina cells of Solen vagina. 



Pigmented cells from epithelium of the eye of Pecten (copied [enlarged] 



from Hensen, Taf. XXI fig. 98/ and/'). 

 Fig. 16. Four retina cells from the siphon of Dreissensia polymorpha (letters as 



in fig. 2 . 



