66 LILLIE. [Vol. X. 



were connected, though one can hardly doubt that such a 

 structure exists. Flemming and Rabl have figured somewhat 

 similar but shorter protoplasmic processes to the bases of these 

 cells. Their reaction to methylene blue seems to me fresh 

 evidence of their sensory nature. 



I do not include a detailed description of the cells in 

 question ; for that has already been done by Flemming and 

 Rabl. I shall merely call attention to the conical form of 

 these cells and their elevation above the surrounding surface 

 (Fig. 92). They are supposed to transmit to the adductor 

 muscle the stimulus which causes its contraction. The neces- 

 sary stimulus might of course come from contact with the 

 prospective host, in which case the contraction of the muscle 

 would force the hooks into its skin, thus securing the requisite 

 attachment (Schierholz). 



Mesoblast. 



In the stage of Fig. 82 and presumably somewhat later, the 

 teloblasts of the primary mesoblast are still recognizable. The 

 remainder of the primary mesoblast has fallen into a clump of 

 small cells, which take up a position behind the entodermic sac 

 as the latter moves forwards (Fig. 84). In the glochidium the 

 mesoblast is very distinctly paired and lies in contact with the 

 posterior walls of the lateral pits (Figs. 89, 94, and 97), stretch- 

 ing to the posterior end of the embryo on each side. A special 

 wing of the mesoblast may be seen on each side behind the 

 lateral pits. In well stained specimens this portion of the 

 mesoblast shows but few clear nuclei with distinct nucleoli 

 (Fig. 93). According to Schmidt, these cells are the funda- 

 ment of the organ of Bojanus (the nephridia). 



Some of the myocytes are specially modified as retractors of 

 the hooks (Schmidt). The others are attached to the larval 

 mantle and shell, and serve to keep the former in varying 

 degrees of approximation to the latter. 



Entoderm. 



In embryos of Unio of the stage of Fig. 82 the entoderm no 

 longer forms a sac, but has become a mass of cells (Figs. 83 



