54 LEECHES 



and excreta. The blood in the longitudinal vessels is a clear fluid 

 in which no respiratory pigment has been identified. The coelomic 

 fluid is colourless in the rhynchobdellids but contains haemoglobin 

 in solution in the gnathobdellids. In the latter group there is 

 no trace of true blood vessels, not even in development. Their 

 circulatory system consists entirely of coelomic sinuses and 

 circulation is brought about mainly by the contraction of lateral 

 sinuses which have acquired muscular walls. 



Oxygen uptake in most leeches takes place through the general 

 body surface, but the Piscicolidae have accessory respiratory 

 organs filled with coelomic fluid which may be large leaf-like gills, 

 as in Branchellion (Fig. 18) or small hemispherical vesicles which 

 pulsate rhythmically {Piscicola^ Fig. 17). When it is necessary to 

 increase the rate of oxygen uptake Piscicola increases the rate of 

 pulsation of its vesicles but other leeches ventilate the body surface 

 by means of dorso-ventral undulations which pass along the body 

 from head to tail. This movement is carried out while the posterior 

 sucker is held fast to the substratum, the head moving freely, and 

 it has been described as a swimming movement performed while 

 standing still. The rate of ventilation or of pulsation is related to 

 temperature in the manner shown in Fig. 28. 



An alternative or supplementary method of increasing oxygen 

 uptake is to increase the rate of pumping of the coelomic fluid 

 round the body. Herter (1936) found that in Erpohdella^ where it is 

 sometimes possible to observe the lateral sinuses by transparency, 

 the rate of pumping varied from 3-7 per min at 17°C to 17-1 per 

 min at 27° C. The mechanism controlling pumping was studied 

 in Hirudo by Gaskell (1919). From each segmental gangHon two 

 nerves run out on each side and send branches to the walls of the 

 lateral sinus. The anterior segmental nerves accelerate the rate 

 of pumping while the posterior nerves cause a retardation. 

 Contractions continue for long periods after all nerves have been 

 severed, suggesting that the stimulus for contraction originates in 

 the muscles of the sinus wall. Such a mechanism is called myogenic 

 to distinguish it from one in which the stimulus originates in the 

 nervous system, which is neurogenic. The arrangement in Hirudo 

 may be compared with that in the mammalian heart, where the 

 beat is myogenic and is accelerated by secondary sympathetic 

 fibres but retarded by a branch of the vagus nerve. The parallel 



