THE NEMATODA. PARASITISM 309 



life (see p. 2) by a remarkable process in which the com- 

 plex molecules of carbohydrates are decomposed to form 

 simpler ones without the intervention of free oxygen. 



To this end such parasites lay up in their tissues large quantities of 

 the starch-like substance glycogen of which carbohydrate stores in 

 animals are usually composed (p. 50). Thirty per cent, of the dry 

 weight of an Ascaris, and nearly half that of a tapeworm, consist of 

 this substance. In Ascaris the glycogen is converted into dextrose and 

 then decomposed according to the following equation : 



4 C 6 H 12 6 = gC0 2 + 9 H 2 + 3 C 4 H 9 COOH, 

 forming carbon dioxide, hydrogen, and valerianic acid. This process 

 yields only 25 per cent, less energy than would be obtained by total 

 oxidation. Thus the animal is "anaerobic" (p. 535). 



It should be noted that parasites are not necessarily 

 harmful. In fact, those which are so have not yet reached 

 that accommodation with their host which the course of 

 generations usually brings about. An organism which 

 suffers injury from a parasite, either by loss of food, or by 

 destruction of tissues, or by poisonous by-products, fights 

 the intruder in various ways — -by expelling it, by attacking 

 it with leucocytes, and by the secretion of counter-poisons. 

 It also lessens the effects of the parasite by taking enough 

 food to supply it as well as itself, and, if poisons be in 

 question, by the secretion of " antitoxins " which neutralise 

 them. It is usually to the interest of the parasite not 

 entirely to overcome these measures, for by so doing it 

 deprives itself of a host. Most parasites have reached an 

 equilibrium with their host, which is able to keep them 

 from destroying, and often even from harming it, but 

 cannot destroy them altogether. 



Allusion has already been made to certain broad resem- 

 blances between the Nematoda and the Arthro- 

 A?thro^da. nd P° da - These mav now be specified. The stout 

 cuticle, shed at intervals, with its underlying 

 syncytium, both inturned in the fore and hind guts, the hae- 

 mocoelic body cavity, the absence of cilia, and the unflagel- 

 lated spermatozoa suggest an affinity between the groups. 

 It is true that the excretory system and some features of 

 the nervous system are entirely peculiar to the Nematoda, 

 and that both segmentation and limbs are wanting. 

 Nevertheless it may well be that these worms are relations 

 qf the Arthropoda highly modified for a peculiar mode of life. 



