TEANSACTIONS OF SECTION D. 



815 



of these cells contains a system of tiny canals running ttrough their cytoplasm 

 (fig. 2', i.d.). These cells are the secreting elements ; they continually cast the silk 

 substance into the tuhe. 



If we suppose that the silk gland was originally composed of a single row of 

 cells, like the nephridium of the leeches, these two difterent structures of the silk 

 glands may be regarded as corresponding to the two stages of the evolution which 

 I have hypothetically indicated for the nephridium. 



(a.) In the case of tenthredinida, the primary cells, after having divided several 

 times in the course of evolution, have been disjoined from one another in such a 



Nepheidia 



Fig. 1. — Leech. 



(Intermediate stage, unknown) 



Fig. 3. — Vertebr.ate. 



Silk Glands 



(Merely intracellular stage, unknown) 



Fig. 2'.— Tenthredinid. 

 'J' 



Fig. 3'.— Lepidopter. 



way that some of them compose the wall of an epithelial excretory canal, and some 

 others remain as appendages to that wall. The first have lost their mternal 

 ductules, the second keep them still, and are the silk-producing elements. 



(b.) In the case of a simple tubular gland, as that of lepidoptera, the cells hove 

 not only divided and separated from one another, but have all lost their internal 

 ductules, and secrete through their inner face only. 



Now, although I give all this as mere supposition, it is desirable to know 

 whether there exists or not among the uephridia any disposition corresponding to 



I 



