148 BULLETIN OF THE BUREAU OF FISHERIES. 



may be fixed with no serious shrinkage by using the solution of plain corrosive sublimate 

 obtained by diluting a saturated solution two or three times with water. Acids should 

 be avoided throughout the whole process. Alum cochineal, Delafield's hEematoxylin, 

 and borax carmine, alone or with Lyon's blue, have been used as stains, each being 

 more suitable for the demonstration of certain structures. In this stained material the 

 shell shows a slight wrinkHng of its ventral flange and is the only part not shown to 

 better advantage than in the living specimens. 



The lateral pairs of sensory cells (fig. 14 and 15, pi. viii) are tall chimney-like 

 structures expanded at the base and terminated by several very fine motionless proc- 

 esses. A denser border where these processes are inserted in the cell is presumably due 

 to their continuation within the cytoplasm which has been observed in sections of these 

 and other glochidia. The two median pairs of sensory cells (fig. 14, pi. viii) project 

 only a short distance and have short processes. The anterior pair is located ventral 

 to the median portion of the larval adductor muscle, the posterior pair near the outer 

 ends of the rudiments of the adult organs (fig. 15, pi. viu). The designation of these 

 cells as "sensory" by all writers rests upon their structural features as described by the 

 earlier investigators, and upon the fact, recorded by Lillie (1895), of their staining 

 reaction with methylene blue. The actual connection of the cells with the larval muscle 

 fibers has been sought for by investigators, but never discovered. We have not 

 attempted a further demonstration of the function of these cells by the methods prac- 

 ticed in recent experimentation upon the protozoa and other minute organisms, although 

 such a study might yield some interesting results. 



Lining the greater part of the surface between the valves, are the large cells com- 

 posing the larval mantle (fig. 15, pi. viii). They are filled with fine granules, which, 

 since these cells actually digest the tissue of the host during the early stages of the 

 parasitism, are probably the zymogen granules from which the digestive enzymes origi- 

 nate. The absence of these cells over the area of flexure ventral to the adductor muscle 

 will be noted in figure 15. In this area the ectoderm is thinner and there is no granu- 

 lation. The adductor muscle is composed of fibers having elongated nuclei and often 

 seen to branch toward the ends where they are attached to the valves. In a glochidium 

 of Lampsilis subrostrata, which has been carried over the winter in the parent gills and 

 which has therefore reached the highest stage of differentiation possible for this glochi- 

 dium, we can identify the rudiments of foot, stomodaeum and enteron, and of the heart, 

 pericardium, and kidney, as described by Harms (1909) in his accounts of the structure 

 and organogeny in the bookless type of glochidium. Reference to figure 15, plate viii, 

 will make clear the following account of these rudiments. 



In the median region, just posterior to the adductor, is a triangular area, the oral 

 plate; behind this a narrow band of closely set nuclei extending well out into the valves, 

 where it becomes wider. The ectoderm in the median part of this area becomes the 

 covering of the foot, while the deeper part of the area is endoderm, the rudiment of the 

 enteron. The lateral expansions of this general mass are mesodermal cells which are 

 closely applied to the endoderm and in which are found the rudiments of the kidney, 



