PSEUDOCOELOMATE AND MINOR EUCOELOMATE PHYLA 



necessary, as well as in the storage of reserves such as glycogen and lipids. 

 Nematodes which are intestinal parasites, or which live amid decaying organic 

 matter, either absorb or ingest food which has already undergone considerable 

 decomposition; this obviates the need for elaborate digestive mechanisms. 



The nervous system is centered in a circumpharyngeal nerve ring, from 

 which nerve cords and fibers extend anteriorly and posteriorly. These fibers 

 innervate the muscle cells and the sense organs. Although this is a rela- 

 tivelv simple system, the coordination of activities which it brings about is 

 remarkable. 



Unlike most other Aschelminthes, the nematodes do not possess proto- 

 nephridial svstems. The function of excretion is served by a variously modi- 

 fied system of cells, sometimes with elongated intracellular canals, termed 

 renettes (Fig. 12.3). These unique cells appear to function by absorbing 

 wastes from the fluid in the pseudocoel and eliminating them at an excretory 

 pore. There is no evidence in anv known nematode for the existence of a 

 protonephridial system; presumably this has been lost in the course of 

 evolution. 



The reproductive mechanisms of nematodes are relatively simple, although 

 the life cycles are often complicated. The sexes are usually separate, and 

 the gonads are tubular organs continuous with their paired, much elongated 

 ducts. In the female the ducts join and open at a separate genital pore on 

 the midventral line. In males the paired condition of the gonads is often 

 lost; the paired or single sperm ducts proceed posteriorly and open into the 

 rectum. Males are usually smaller than females and are provided with spines, 

 called penial setae, which serve in copulation. In most marine and terrestrial 

 nematodes there appears to be a tendency toward suppression of the male 

 sex; many species show a large preponderance of female individuals. This is 

 apparently not the case in fresh-water or parasitic forms. 



Like the rotifers (p. 348), the nematodes are characterized by constant 

 numbers of cells in their organs, or by constant numbers of nuclei in syncytial 

 structures. Except in the germinal tissues, mitotic activity ceases at maturity, 

 after a particular number of muscle cells, gut cells, etc., have been produced. 

 In many of the larger parasitic nematodes there is a subsequent increase in 

 the numbers of cells or nuclei as the worm grows, but this involves some kind 

 of amitotic nuclear fragmentation. 



Parasitic Nematoda: Ascaris lumbricoides. The adult worm, inhabiting 

 the small intestine of man, is morphologically indistinguishable from the 

 ascaris of swine and many other mammals. Larvae of the hog ascaris will not 

 develop in man, however, and vice versa; therefore the hog ascaris is con- 

 sidered a separate subspecies or variety of A. lumbricoides. The female of 

 this species (Fig. 12.4) is 8 to 16 inches in length, the male 6 to 8 inches. 

 The overall characteristics of the species are similar to those described for 

 a generalized nematode in the preceding section. In feeding, the intestinal 

 contents of the host are drawn into the muscular pharynx of the worm and 

 passed into its intestine. Here, absorption of this predigested mixture occurs; 



351 



