694 PROCEEDIXGS OP THE ACADEMY OF [DeC. , 



Just beneath the outer epithehum is a layer of very fine muscle 

 fibres, Ep.m.f., running circularly around the proboscis. These 

 may be termed the subepithelial circular muscle layer'' of the pro- 

 boscis. In a cross section of the proboscis, fig. 35, these fibrils 

 are in longitudinal section, but in a longitudinal section of the 

 proboscis, fig. 38, they are cut transversely, ana it may be seen 

 that the layer is only one fibril thick. 



The basement layer, or ground substance, is a homogeneous, 

 gelatinous-looking structure of considerable thickness. It is a pro- 

 duct of the fibres of connective tissue cells, and their nuclei may 

 be found scattered here and there among the fibres. The thick- 

 ness varies considerably in different specimens. It may be most 

 favorably studied in a specimen fixed with Gilson's fluid, one of 

 the best fixatives for connective tissue. The region of the greatest 

 thickness is always in the anterior part of the proboscis and rapidly 

 diminishes toward the middle region. 



The longitudinal muscle layer, L.M., consists of bundles of fibres, 

 about eight to ten fibres in each. 



The two jiroboscis nerves, P.N., are distinct from one another, 

 each surrounded by a sheath of connective tissue. 



The structure of the inner basement layer, i.B.L., is similar to 

 that of the outer layer but is only about one-third as thick. The 

 inner epithelium, i.Ep., is a one-celled layer. The cells are square 

 in cross section, the cell-membranes distinct and the nuclei large 

 and rounded. Between the epithelial cells are numerous gland 

 cells, Gl.^, the contents of which stain dark blue with hsematoxy- 

 lin. The central cavity of the proboscis is quite large in this 

 region. 



The transition stages between the anterior and middle regions 

 are characterized by the decrease in thickness and the almost total 

 disappearance of the outer basement layer. Its diminution is 

 coincident with the establishment of a circular muscle layer on 

 the axial side of the longitudinal muscle. A considerable amount 

 of connective tissue appears Avithin the new circular muscle layer, 

 forming the core of the papilhc into which the inner surface 

 of the proboscis is now raised. A considerable increase in the 



® The subepithelial muscle fibrils are uot seen in fig. 14, which represents 

 a portion of the proboscis only a few sections behind its point of attach- 

 ment, but they begin a short distance farther back and I'orni a continuous 

 layer to the posterior tip of the proboscis. 



