No. 6.] PARASITISM OF ANODONTA PLANA LEA. 305 



is, however, the only one found after opening dozens, if not 

 hundreds, of Unios. 



a. 



II. NATURE OF THE COLORATION. 



In order to determine, if possible, the exact relation of the 

 color deposit to the shell, I made a vertical section of the shell, 

 by sawing out a piece and then grinding it down to the neces- 

 sary thinness on a stone. A view of this section is given in 

 Fig. 2. The three usual layers of the 

 shell are seen ; viz., the epidermis, on 

 the outside ; the prismatic layer beneath 

 this, in which vertical prisms are dis- 

 tinctly visible ; and the inner nacreous 

 layer. The nacreous layer, however, 

 is rather distinctly divided into two por- 

 tions, a deeper, normal layer closely 

 laminated and not colored, and an outer 

 layer chiefly composed of the same 

 material, but containing the red colora- 

 tion in addition. The red is not a 

 separate and distinct material mixed 

 with an equally separate nacreous stuff, 

 but the two are so intimately blended 

 that it is impossible to show the red 

 coloration in a black and white draw- 

 ing. It thus appears to be rather a 

 modification of the nacre, or at any 

 rate a material dissolved in it, than any separate deposit. 



Surface views of the red deposit were also studied. They 

 were obtained by first decalcifying the shell in dilute hydro- 

 chloric acid, after which the lamination of the nacreous portion 

 became very distinct and permitted me to tear off very thin 

 pieces of considerable size. Such pieces under the microscope 

 do not show any distinct colored material ; they are structure- 

 less membranes which either are colorless or else they are 

 salmon tinted. But in no case was I able to find any distinct 

 and separate deposit. These facts are correlated with the 



d. 



.01 IN. 



FIG. 2. Cross-section of shell of 

 Anodonta through a thick deposit 

 of red material, showing its na- 

 creous character, a = epidermis; 

 b prismatic layer; r = nacre; 

 d = deposit due to the fluke. 



