226 
tatives of this group than has ever been possible heretofore.. Professor 
H. B. Ward has placed his collections and records from Havana, Illinois, 
at my disposal. Mr. R. H. Linkins has permitted the publication of two 
specific definitions which have previously occurred only in manuscript, 
concerning which a fuller statement is given later in this paper. I am 
especially indebted to Dr. G. R. La Rue for numerous collections of speci- 
mens including representatives of two new species. Collections of the 
U. S. National Museum and of the Bureau of Animal Industry, received 
through the courtesy of Dr. C. W. Stiles and Dr. B. H. Ransom, with the 
private collections of Dr. A. S. Pearse and of Dr. A. R. Cooper, have 
furnished an abundance of material for comparative study. 
Hasits oF THE ACANTHOCEPHALA 
THE LIFE CYCLE ¥ 
The present paper deals with one of the most highly specialized 
groups of animal parsites in its relations to its hosts. The Acanthocephala 
are a group of worms the individuals of which reach sexual maturity in 
the digestive tract of various vertebrates. Life histories are not known 
with certainty for any of the species found in fresh-water hosts of North 
America, but there is no evidence that they ever lead an independent 
existence even for the shortest periods of time. Their development has 
been studied in a number of European species. In all instances it has been 
found that the embryos produced by the females never lose their resistant 
confining membranes after they are set free from the body of the defini- 
tive host until these embryos are taken into the body of the primary host. 
The primary host is usually an arthropod. Embryos of the Acantho- 
cephala are taken into the digestive tract of the arthropod along with 
food. Here they are liberated from their confining shells and undergo 
further development within the body of the primary host. Intermediate 
hosts are not infrequent in the life cycle of the Acanthocephala. If pri- 
mary hosts bearing larval Acanthocephala are eaten by an animal in 
which the larvae are unable to complete their development, the larvae 
become encysted in the tissues of the new host. The entrance into the 
definitive host is, in this instance, contingent upon the intermediate host’s 
serving as food for the definitive host. The parasite never reaches sexual 
maturity unless the primary or intermediate host is eaten by a vertebrate 
in which the worm is capable of continuing its development. Thus the 
parasite is in every step of its development dependent upon some other 
organism for its maintenance. This absolute dependence of the Acantho- 
cephala makes a study of their interrelationships with the host a topic of 
considerable economic importance. This is true even though the host 
affected may not be of direct commercial value to man. The interde- 
pendence of life in the same habitat has been so frequently emphasized 
that it need not be discussed fully here. One example will serve as an 
illustration. The gizzard-shad (Dorosoma cepedianum) has practically no 
direct commercial value, yet it serves to such great extent as food for the 
