344 



HISTOLOGY 



excretory cells the calcium phosphate cells which Barfurth so names 

 and describes for Helix, We shall take as our example those found in 

 the hepato-pancreatic epithelium of Mesadon. It is interesting to note 

 in this connection that the concretion particles found in the renal sacs 

 of Nautilus contain, according to Keferstein, cal- 

 cium phosphate bodies and other salts, but no 

 urine. 



In the vertebrates and some invertebrates, well- 

 developed conducting channels and retaining ves- 

 sels for urine are developed. These are the ureter, 

 bladder, and urethra. 



The sweat-glands elaborate a certain amount 

 of urine and other waste products. These struc- 

 tures have been considered in connection with 

 the integument. It is of interest to point out at 

 this place that they too are epithelial in charac- 

 ter. 



Nephridial tubules. The nephridia of the tape- 

 worm of the robin. The mesenchyme of a tape- 

 worm contains many intercellular spaces filled with 

 a body fluid. Into these passages waste solutions 

 are emptied. This waste is taken from the body 

 fluid by numerous excretory cells which deliver 

 the collected material to a system of excretory cap- 

 illaries. The walls of the capillaries are com- 

 posed of very thin tile- or plate-like mesenchymal 

 cells. The ultimate branches of these capillaries 

 bear the excretory cells. These cells have a cyto- 

 plasmic body which develops a collar (Fig. 303). 

 The collar forms the terminal part of the wall of 

 the excretory capillary. Into the lumen of this collar the terminal cells 

 send a tuft of heavy cilia which in life flickers in a manner that has sug- 

 gested the term "flame" for it. A large rounded nucleus lies in the 

 distal part of the cytoplasm (Fig. 303, nu.}. This type of cell is known 

 as a "flame-cell" or solenocyte. 



The nephridium common to insects is a blind tubule known as a Mal- 

 pighian tubule. The Malpighian tubule of a caddis-fly larva is simple 

 and unbranched. The wall is composed of two rows of grooved cells 

 that have almost lost their individuality to form a syncytium. The 

 nuclei on one side of the tubule alternate with those on the other. They 

 are large, and have their chromatin uniformly distributed as spherical 

 granules. The cytoplasm is rather dense and homogeneous. The 

 inner margin bears a less opaque striated cuticula. The lumen is rela- 



nu,. 



FIG. 303. A flame-cell 

 from the tapeworm 

 found in the robin. ., 

 nucleus; c., collar; ., 

 neck. Mass of cilia 

 inside. 



