32 ILLINOIS BIOLOGICAL MONOGRAPHS [136 



ably seen when a comparison is made of the fascinating and intricate 

 structure of the head region of free living nematodes with the gross and 

 generally monotonous anatomy of that portion of the parasitic species. 

 Sensory bristles, cephalic setae, ocelli, and other sensory structures as 

 well as the amphids found in free forms are among the first organs to 

 disappear after the assumption of parasitism. Then the digestive system 

 is progressively altered most noticeably in the cephalic region, viz. the 

 oral and pharyngeal structures. Concomitant with the general simpli- 

 fication and loss of structures, there is a rather progressive hypertrophy 

 and complication of the reproductive systems, because as parasitism in- 

 creases the chances for propagation are less favorable. Further adapta- 

 tions are seen in the production of cuticular cephalic expansions, as the 

 lateral alae of Oxyuris tetraptera von Linstow, the cuticular bosses of 

 Gongylotiema musculi Neuman, the ventral cuticular combs or spines of 

 Rictularia, or the hook beset head of Echinocephalus, etc., which are 

 only a few of the possible variations. In a general way, these outgrowths 

 are adaptations which form excellent hold fast organs, as they are found 

 only among those species which inhabit the alimentary canal. 



At this point it might be well to consider the possibilities of specializa- 

 tion of the nematode parasite with respect to its habitat. Specialization 

 among the parasitic species is usually the opposite of that among free 

 living forms, for with few exceptions (hold fast organs and reproductive 

 organs) the term implies a simplification or a loss of existing structures. 

 The roundworms living in the posterior and anterior portions of the alimen- 

 tary system of their host more nearly approach the free living forms than 

 any others. For example, those living in the ceca and large intestine 

 (Oxyuris, Ascaris) live largely upon the bacterial flora because most of the 

 split proteins and carbohydrates of the digested food have been removed 

 from the intestinal contents by absorption long before these regions are 

 reached. In the stomach little actual hydrolysis of the food is accom- 

 plished so that nematodes inhabiting this organ must use and digest to a 

 large extent the food presented them by the host. This fact is demon- 

 strable in Protospirura muris inhabiting the stomach of the common mouse, 

 for in this species, the intestine is usually filled by minute fragments and 

 starch granules derived from the host's diet of grains. Other species such 

 as those of Ancylostoma actually feed upon the cells of the intestinal papil- 

 lae according to the observations of Looss. 



From a parasitic point of view, the most highly specialized nematodes 

 are those inhabiting the circulatory system (Filaria), the body cavity and 

 the connective tissues (Gongylonema and Dracunculus). They must 

 needs depend for their nourishment upon the absorbed food products 

 circulating in the blood and lymph with which they are bathed and from 

 which they osmotically acquire the necessary elements for their own me- 

 tabolism. One would expect in such a case to find a correlation between 



