GAMMARUS. 343 



do not themselves contain food, but they form a secretion 

 which passes down their wide lumina to be poured into 

 the mid-gut, and probably also into the stomach. Two 

 longitudinal plates of cuticle, thickly studded with setse 

 (PI. IV., fig. 2, set. cut J) pass from the stomach backwards 

 along the floor of the sac from which the hepato-pancreatic 

 diverticula arise. These partially enclose a channel 

 which is continuous with the ventral groove of the 

 stomach, and probably serves to carry forwards into the 

 latter the hepato-pancreatic secretion. 



As Murlin (13) found in the Isopoda, the walls of the 

 hepato-pancreatic tubes show four layers : — (1) a serous 

 membrane, (2) muscle fibres collected into a more or less 

 distinct band taking a spiral course, (3) a fine basement 

 membrane, (4) an epithelial layer. As Weber (25) pointed 

 out, the spiral muscle band ensures greater efficiency of 

 the peristaltic wave in discharging the contents of the 

 tubes. The epithelial layer consists of tall, conical cells 

 projecting considerably into the lumen, and separated by 

 lower, non-projecting cells (PI. IV., fig. 1, h.p.). For 

 some distance behind the point of origin of the tubes, as 

 well as near their ends, the two forms of cells merge into 

 one another, and are probably, contrary to the supposition 

 of Weber, merely the same kind of cell in different stages. 

 The tall cells are much distended by colourless globules, 

 the contents of which are emptied into the lumen of the 

 tube by the bursting of the cell, which then appears much 

 shrunken in size. Cells may be observed containing 

 globules of every intermediate size, the globules 

 being formed on that side of the cell near the 

 lumen of the tube, the nucleus lying nearer the basement 

 membrane. The mid-gut itself consists of four layers : — 

 (1) a serous membrane, (2) a very thin muscular layer, 

 consisting chiefly of transverse fibres, as Wrzeniowski 



