al2 Annals Entomological Society of America [Vol. VII, 
Incidentally, the behavior of the digestive canal of insects 
is only imperfectly known, and though I have nothing to add 
either to the various cell-structure theories or to the theories 
concerning digestion, yet these notes recording what seem to be 
the actual histologic conditions of the digestive epithelium in a 
number of insects, and suggesting some possible significance of 
these conditions, may have a little interest. 
First, let us take up the matter of cell structure and cell 
relationship. The old idea that cells were like so many bricks, 
each to be considered separately, has had to be discarded; and a 
multicellular organism can no longer be compared to a brick 
building. At first sight this may seem to be a proper com- 
parison, but reflection will show that the differences are great. 
For the cells of an organism are by no means constant; they 
are continually being built up and destroyed. A cell is injured, 
or it completes the work for which it was created: it disappears, 
and a new one takes its place. Cell walls are not mere rigid 
boundaries; they are elastic, permeable, capable of radical 
change. These things all go hand in hand with a specialization 
and consequent interdependence of cells, which makes impera- 
tive a study of the behavior of many cells rather than of single 
cells. 
A study of insect digestive cells soon makes it apparent that 
here a definite and constant cell structure does not exist. Diges- 
tive cells are extremely active. Two important types of cell 
secretion may be distinguished, called by Haseman the holo- 
crine and merocrine types. In the former the whole cell con- 
tents is discharged at once, and new cells arise. In the latter 
the discharge is gradual and continuous and the cell remains. 
active for along period. I shall take up this matter of secretion 
more fully in connection with the subject of digestion. The 
important thing here is to note the marked difference in be- 
havior between the two types of cells. 
The holocrine type is very well represented by the digestive 
cells of the dragon-fly (Needham), or by those of almost any 
predaceous beetle (Fig. 6). Here the cells are formed in nests 
or nidi, using Needham’s term, and gradually develop and 
increase in size until they are capable of secretion. Upon the 
introduction of stimulus in the form of food, the largest cells 
burst, pouring out their contents to mix with the food, and the 
