Host-Parasite Relationships 531 



observed in the haemocoel of insects (29, 34, 67, 93), in the gills of 

 Gammanis pulex (78), in the haemocoel of crabs (79), in the coelom of 

 sea-urchins (64), and in the digestive tract of slugs (82). Such temporary 

 invasions may be comparable to the initial step in the origin of endo- 

 parasitism. 



The transition from sporadic invasion to the establishment of natural 

 parasitism need not have required any marked morphological changes. 

 This is obvious in many parasitic species which belong to predominantly 

 free-living groups. Both free-living and parasitic species sometimes occur 

 within a single genus. Species of Astasia have been reported as parasites 

 of rhabdocoeles (5), rotifers (95), and Crustacea (1), although others are 

 free-living. Eiiglena leucops (35), parasitic in a rhabdocoele, has lost its 

 chlorophyll but resembles free-living Euglenidae in other respects. 

 "Astasia" chaetogastris, found in lethal infections of an oligochaete, re- 

 tains the stigma but discards the flagellum of the free-living stage (23). 

 Another example, Euglenamorpha hegneri (97), occurs in the rectum of 

 tadpoles as two varieties, one with chlorophyll and the other without. 

 Loss of chlorophyll can scarcely be considered an adaptation to parasit- 

 ism, since the same change has occurred in free-living Euglenidae exposed 

 to darkness and other experimental conditions. Hexamita apparently 

 represents an extreme case in which free-living species and parasites of 

 the digestive tract in various invertebrates and vertebrates have been 

 assigned to one genus. Likewise, members of the ciliate genera Anophrys, 

 Colpidium, Colpoda, Metopus, and Uronema, which include mostly free- 

 living species, have been reported as intestinal parasites of sea-urchins. 

 Even such genera as Balantidium and Nyctotherus, which include para- 

 sites only, cannot be distinguished from free-living ciliates by morpho- 

 logical criteria. Obviously, the initial stages in development of endo- 

 parasitism do not demand appreciable changes in structure. Accordingly, 

 it may be assumed that the primary adaptations have been physiological 

 rather than morphological. 



There are, however, parasites which have undergone more or less 

 extensive morphological specialization, and thus seem to show structural 

 adaptations to parasitism. The absence of feeding organelles in the 

 Opalinida and Astomina is sometimes considered an example of regres- 

 sive evolution in ciliates, which are predominantly holozoic organisms. 

 However, the loss of holozoic habits is not a universal feature of special- 

 ized parasites. The Cycloposthiidae and Ophryoscolecidae, for example, 

 include highly differentiated ciliates which are distinctly holozoic. Or- 

 ganelles of attachment have appeared in such parasites as gregarines, the 

 peritrich Ellobiophyra donacis (18), various dinoflagellates (16, 74, 89), 

 and such termite flagellates as Streblomastix (59), and Microrhopalodina 

 (Proboscidiella) (50). Another common feature is the occurrence of rapid 

 multiplication at certain periods in the life-cycle, as in merogony and 



