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instances the number of individuals examined for these parasites is too 
small to yield reliable data in this direction. One noticeable feature of the 
extent of infestation in this region should not be passed without com- 
ment. This is the adaptability of certain species to various hosts. It has 
been found that while a given species of parasite occurs in large num- 
bers in certain host species it has been found occasionally in small num- 
bers, frequently singly, in the intestine of different host species. This 
observation is especially true for the distribution of Echinorhynchus 
thecatus Linton. In a number of instances a single individual of this 
species has been found as a result of the examination of a fairly large 
number of fish of the same species, while practically every bluegill 
revealed a relatively large number of these worms. There are but few 
evidences of a fixed specificity of hosts such as are found in other para- 
site groups. In most instances differences in frequency of occurrence of 
these parasites are to be sought in differences in food habits of the hosts. 
Since the definitive host secures its Acanthocephala only through feeding 
upon the infested primary or intermediate host, the degree of infestation 
ot the definitive host must be in some way correlated with the extent to 
which it preys upon the hosts of the larval parasite. - 
INFLUENCE oF AGE or Host on INFESTATION 
Difference in degree of infestation within the same host species is 
frequently influenced by the age of the specimens of the host examined. 
The writer has frequently observed that young and very small fish may 
be free from acanthocephalan infestation even though the larger and pre- 
sumably older specimens of the same species regularly carry parasites. 
The explanation of this difference has usually been sought in change of 
food habits by the fish at different ages. In the specimens of the gizzard- 
shad examined by the writer the negative records are due in many 
instances to periodicity in the occurrence of the species of Acantho- 
cephala infesting this fish (Van Cleave, 1916), but it has also been 
observed that small individuals of this species rarely reveal an infesta- 
tion. In the light of the food habits of the gizzard-shad the reverse might 
well be expected. The food of the young of this species, according to 
observations by Forbes and Richardson (1908 : 47), consists “almost 
wholly of small crustaceans and insect larvae” while that of larger speci- 
mens comprises “quantities of mud, with which the intestine is com- 
monly packed from end to end, mixed with many minute plants, and much 
vegetable debris.” The present writer has also observed that macroscopic 
animal forms are but rarely represented in the stomach contents of larger 
specimens. In spite of the fact that the young of this species feed almost 
exclusively on small arthropods which might serve as primary hosts of 
Acanthocephala, they are rarely infested, while the larger specimens, 
which are to a great extent vegetable and detritus feeders, usually yield 
large numbers of Acanthocephala. 
