36 



columnar epithelial cells, with an oval nucleus situated 

 near the middle of the length. A distinct border, due to 

 the desmochondria at the base of the cilia, is present. 

 With either methyl-blue-eosin, or Mallory's stain, the 

 contents of the eosinophilous cells (small spherical 

 granules) (fig. 24, Ev. c.) stain an intense red and the 

 contents of the gland cells light blue (fig. 24, Gl. c). 



The most interesting part of the pharynx is, however, 

 its muscular sheath, which underlies the epithelium. It 

 may be divided into two layers, longitudinal fibres 

 situated next to the epithelium, and an outer circular 

 muscle layer (fig. 24, Mus. long, and Mus. circ). There 

 is no outer layer of epithelium bounding the cavity in 

 which the pharynx lies, for this is simply a haemocoele. 

 The muscle fibres are extremely thick, and each is 

 surrounded by a capsule of connective tissue. This 

 matrix is, however, so reduced that it has rather the 

 appearance of very thick cell walls, where the muscle 

 fibres are cut transversely. The fibres themselves 

 resemble the cell contents, for they are almost round in 

 transverse section ; the angular shape really possessed by 

 them is probably due to crowding and pressure. The 

 structure of the fibres is extremely distinct in this 

 pharyngeal musculature, and one sees a beautiful 

 peripheral arrangement of fibrils surrounding a large 

 central granular sarc, in which may sometimes be seen 

 the nucleus. A better or more easily procured example 

 of this type of muscle fibre could hardly be imagined. 

 These large fibres with their great sarcoplasmic centres 

 give the reddish pink tinge to the pharynx, a colour which 

 is hardly ever present in molluscan muscles, and in the 

 whelk in one other place only, the muscles of the 

 odontophore. 



The Gland of Leiblein. — So far as I am aware, no 



