4 THE ANIMAL PARASITES OF MAN 



The new characters which permanent parasites may acquire are, 

 first of all, the remarkably manifold CLASPING and CLINGING ORGANS, 

 which are seldom (as in parasitic Crustacea) directly joined on to 

 already existing structures. In those instances in which organs 

 for the conveyance of food are retained, these likewise frequently 

 undergo transformation, in consequence of the altered food and 

 manner of feeding. Such alterations consist, for instance, in the 

 transformation of a masticating mouth apparatus into the piercing 

 and sucking organs of parasitic insects. 



HERMAPHRODITISM (as in Trematodes, Cestodes, and a few Nema- 

 todes) is a further peculiarity of many permanent parasites ; moreover, 

 the association in couples that occurs, especially in trematodes, may 

 lead to complete cohesion and, exceptionally, also to re-separation 

 of the sexes. In many cases the females only are parasitic, while 

 the males live a free life, or there may be in addition the so-called 

 complementary males. Occasionally the male alone is parasitic, and 

 in that case lives within the female of the same species, which may 

 live free, like certain Gephyrea (Bonellia) or the female also may 

 be parasitic, as Trichosoma crassicaudiwi, which lives in the bladder 

 of the sewer rat (Mus decumanus). 



, We have numerous proofs that demonstrate how considerably 

 the original features of many parasites have become changed. We 

 need only draw attention to the aforementioned Linguatulidae, also 

 to many of the parasitic Crustacea belonging to various orders. In 

 all of these a knowledge of the larval stages in which there is no 

 alteration, or at most only a slight degree of change serves to 

 determine their systematic position, i.e., the nearest conditions of 

 relationship. 



The most remarkable changes are observed in those groups that 

 contain only a few parasitic members, the majority leading a free 

 life. A striking instance is afforded by a snail, the well-known Ento- 

 concha mirabilis, M tiller. This mollusc consists merely of an elongated 

 sac living in a Holothurian (Synapta digitata). It possesses none 

 of the characteristics of either the Gastropoda or any molluscs, 

 and in its interior there is nothing to be observed but the organs 

 of generation and the embryos. Nevertheless, the Entoconcha is 

 decidedly a parasitic snail, as is clearly proved by its larvae, but 

 it is a snail which, in consequence of parasitism, has lost all the 

 characteristics of molluscs in its mature condition, but still exhibits 

 them in the early stages of development. 



Certain nematodes show very clearly to what devious courses 

 parasitism may lead. The Atractonema gibbosiun, the life-history of 

 which has been described by R. Leuckart, and which lives in the 

 larvae and pupae of a dipterous insect (Cecidomyia), exhibits, in its 



