62 



LIFE-HISTORY OF PARASITES. 



and in this way undergoes a metamorphosis which leads sooner or 

 later to its definitive form. At the same time the parasite fixes 



Fig. 43. — ShaicUtU-'like condition of young stage of 

 Fig. 42. — Embryo of Rhabditis Dochmius trigonocephalus; a, at commencement of the 

 terricola. free life ; b, at the end of the free life. 



itself on to the outer skin of its host, or in some organ easily accessible 

 from the exterior. In this way we know that the larvae of 

 Trematodes attach themselves to the skin or within the respiratory 

 cavities of water-snails. Others bore their way at once into the 

 intestines or body-cavity. To attain this the parasite seeks a soft, 

 slightly resisting part of the body, against which it presses with its 

 anterior extremity, and graduUy forces its way in. Considering the 

 small size of the body, and the fact that many of these embryos are 

 provided with special boring apparatus — as, for instance, the larvae of 

 Bothriocephalus, Gordius, and several species of Distomum — it is evident 

 that the difficulties to be overcome are not very great, provided that 

 they attack the right host. It is of course only animals with 

 a delicate outer skin, such as larval Insecta, Crustacea, Mollusca, 

 and so forth, that are attacked in this way by parasites. 



In many cases the process just described has been actually 

 observed, and in other cases it is inferred by placing together the 



Digitized by Microsoft® 



