972 PROTOZOA AND OTHER ANIMALS 



A further problem arises in the absorption of the substances released 

 from the flagellates by the termite or roach tissues. Either absorption 

 must take place through the chitinous layer of the hind-gut, or fluid 

 must be passed forward into the mid-gut. The problem has been dis- 

 cussed by Buchner (1930) and Cleveland et al. (1934). There are dif- 

 ferences of opinion as to whether absorption in the hind-gut is possible, 

 and some authors are inclined to the view that it is. Buchner is one of 

 those. Abbott (1926) found that the hind-gut of Periplaneta aus- 

 tralasiae is permeable to dextrose. Cleveland et al. took the opposite 

 view, as a result of osmotic experiments on the colon of Cryptocercus, 

 which showed it to be impermeable to dextrose and water. The peri- 

 trophic membrane also seemed to be largely impermeable to dextrose. 

 The iliac valve controls the passage of materials between the mid-gut 

 and the hind-gut, and when the mid-gut is severed it permits no ma- 

 terial to flow out from the hind-gut. Cleveland concluded that fluid con- 

 taining dextrose passes forward at times through the iliac valve into the 

 space between the peritrophic membrane and the wall of the mid-gut. 



A problem in the metabolism of xylophagous animals is the source 

 of nitrogen. The small amount of protein present in wood, and the 

 larger amount in straw and hay (important in the case of Hodotermes 

 and some higher termites), may account for the nitrogen metabolism 

 of the Protozoa; but in the absence of action on cellulose outside of the 

 Protozoa it might not be directly available to the termite. Bacteria and 

 molds ingested with the wood might account for some, but that would 

 probably be very little. Pierantoni (1937) hypothesized a fixation of 

 nitrogen by bacteria in the gut, and Green and Breazeale (1937) re- 

 ported the isolation of nitrogen-fixing bacteria from an unidentified 

 species of Kaloter))ies. Wiedemann (1930) stated that bacteria in 

 certain lamellicorn larvae can use inorganic nitrogen, and the host satis- 

 fies its nitrogen need by digesting these microorganisms. Use by termites 

 and Cryptocercus of some dead Protozoa or pieces of cytoplasm from 

 their bodies (Cleveland et al., 1934), while it could not account for any 

 important part of general nutrition (p. 961), might be significant in 

 providing nitrogen. 



We have seen in the foregoing discussion that many animals ingest 

 substances of which cellulose is an important constituent. In the diges- 

 tive tract of some of them cellulose is broken down, whereas certain 



