4 THE ANIMAL PARASITES OF MAN 



larvse of a great many Trematodes, the oncospheres of some 

 Cestodes). Amongst the retrogressions, the loss of the organs of 

 sense may be mentioned, particularly the eyes, which are still 

 present, not only in the nearest free-living forms but also in 

 the free-living larvae of true parasites ; it is only quite excep- 

 tionally that the eyes are subsequently retained ; as a rule they are 

 lost. Lastly, in a great many cases the digestive system also 

 disappears, as in parasitic crustaceans, in a few nematodes and 

 trematodes, in all cestodes and acanthocephala. There remains at 

 most the rudiments of the muscles of the foregut, but these are 

 adapted to entirely different uses. 



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 crustaceans) 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 living together 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 Trichosomum crassicaudum, 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 crustaceans 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 Enlo- 

 concha mirabilis, Miiller. This mollusc consists merely'of an elongated 



