56 LEECHES 



hearts are neurogenic and the principal accelerator is acetylcholine. 

 Gnathobdellid leeches are clearly exceptional in this respect, and 

 this is further evidence that their circulatory system is not homo- 

 logous with the blood system of other annelids. The source of the 

 adrenalin in leeches appears to be the ganglia of the ventral nerve 

 cord, for Gaskell (1919) demonstrated the chromaffin staining 

 reaction in certain cells of the ventral ganglia and identified 

 adrenalin in ganglion extract. Perez (1942) confirmed this result, 

 although he thought that the adrenalin was more localized in its 

 distribution than Gaskell had suggested. 



Leeches are predominantly freshwater animals, but many fresh- 

 water habitats contain water which is only partially air-saturated. 

 There have been several investigations into the rate of oxygen up- 

 take by leeches at various oxygen concentrations of the water. 

 Early experiments with Hirudo (Lindeman, 1932, 1935) involved 

 placing the leeches in water in a closed container. Samples of 

 water were drawn off at intervals and from the rate of fall of 

 oxygen concentration the oxygen uptake of the leeches was calcu- 

 lated. It appeared that Hirudo was able to maintain a constant rate 

 of oxygen uptake independent of the oxygen concentration down 

 to about 10-20% air-saturation, the precise level depending on the 

 temperature. Similar experiments by Hiestand and Singer (1934) 

 and by Sgonina (quoted by Herter, 1936) gave rather diflFerent 

 results. These discrepancies were probably due to variation in the 

 state of nutrition, time of year and degree of acclimatization. 



In more recent work (Mann, 1956) these variables have been 

 eliminated as far as possible. In Fig. 29 is shown the oxygen con- 

 sumption of five species of leech at various oxygen concentrations. 

 In these experiments the leeches were placed in a small container 

 with water of a known oxygen concentration and left for about 

 1 hr, during which time the oxygen concentration fell by a relatively 

 small amount. From the final oxygen concentration it was possible 

 to determine the oxygen consumption at the mean concentration 

 of the experiment. To build up the curves shown in Fig. 29 large 

 numbers of such experiments were carried out, each point on the 

 graph being the mean of at least five determinations. The highest 

 rate of oxygen uptake in air-saturated water is shown by Piscicola 

 geometra, the piscicolid fish parasite. This leech has to be 

 very active indeed when it is seeking a host and is provided with 



