460 Benjamin Sharp 



the general resemblaiice to other forms where the point is settled 

 beyond questiou of doubt. 



Fani. Myttlidae. 



Mijtilus eduUs. I bave iinfortimately uot had opportunity to ob- 

 serve whether thìs animai retracts the edge of the mantle when a sha- 

 dow passes over it , but the charaeter of the pigmeuted cells that line 

 the edge of the mantle so agree with those forms that bave vision, that 

 I do not hesitate to say that vision exists , especially as this is one of 

 those forms stated by Will to possess highly complex eyes. 



The mantle-edge in this species is not fringed with tentacles , but 

 is thrown into folds between whieh the pigmented cells are found. Into 

 the most exterual fold as in Ostrea extends the ciiticula , whieh bere is 

 much thicker than in Ostrea ; at the base of this fold is attached a 

 musale, the retractor. The extensor muscle is also present, but is not 

 so distinetly marked off into bundles as that described for Ostrea. The 

 circumpallial nervo is situated a little distance from the base of the 2. fold. 



The epithelial covering may bere also be divided into two catego- 

 ries. Those whieh form the nacre of the shell situated on the external 

 face of the mantle and the more distinct columnar cells among whieh 

 are found the pigmented ones, in relatively the same proportion as in 

 Ostrea. 



These pigmented cells are much longer than those of Ostrea being 

 about four times as long but not much broader (comp. fig. 2 and 3) *. 

 They bave a broad base whieh is directed outwards, then tapers to nearly 

 a point where they rest on the tunica (fig. 3 {. . The pigment , whieh 

 is of a dark brown color , occupies about one fifth to one sixth of the 

 celi. The nuclei are large and distinct and are situated in the granular 

 protoplasm of the celi near the pigment , that is , in the upper second 

 fifth of the celi a little above where the commencement of the tapering 

 begins. 



We find in Mijtihcs as in Ostrea between the pigmented cells an- 

 other set of cells , whieh bave their broad ends resting on the tunica 

 (fig. 3 t) , while the narrow pointed end extends up between the large 

 bases of the pigmented cells. Here also as in Ostrea I could not satisfy 

 myself, whether these pointed ends were pigmeuted or not. This state 

 of aflfairs seems only to exist when the seat of vision is on the mantle- 



1 The figures 2, 3, 5, 8, 9 and 16 are ali drawn with a camera lucida of Zeiss 

 with obj. Immersion No. 3 and ocular No. 3., in order they would ali te on the 

 same scale to compare the sizes. 



