k PARASITES OF ANIMALS. 57 



ally more and more like its grandparent. It finally fills up 

 the whole interior of the body of the enclosing embryo, sexual 

 organs appear, and then the outer skin of the latter is cast off*. 

 Thus the young Echinorhynchus. is formed, but in order to be- 

 come mature it must be swallowed, with its crustacean host, 

 by some fish. In the intestine of the fish it becomes sexually 

 mature in about a week. 



The giant Ecliinorhynchus of the hog (^JE. gigas). Figure 74, 

 is the only species known to infest domestic quadrupeds. 

 IV. NEMATODES. (Round-worms). 



This order includes a great number of worms, which occur 

 in all classes of animals, both vertebrates and invertebrates, 

 together with some that are not parasitic, but live in water, 

 moist earth, or decaying animal or vegetable matters. They 

 are much more highly erganized than those of the preceding 

 orders. The pin -worms of children, the common round-worms 

 of man and domestic animals, and the notorious Trichina spi 

 ralis are examples of this order. (See figures 76 to 83.) 



These worms are almost without exception long, round, and 

 rather slender, varying in length from a tenth of an inch or 

 less to three feet, or even more, and usually tapering to one or 

 both ends. The external surface of the body Js generally 

 smooth and uniform, without suckers or other appendages. In 

 the interior of the body there is a continuous visceral cavity, 

 containing a distinct digestive system, the intestine generally 

 extending from one end of the body to the other, and well de- 

 veloped reproductive organs, the sexes being separated. The 

 mouth is usually at the smaller end of the body and is often 

 surrounded by papillae or other organs. Some of the species 

 are viviparous, like Trichina ; others produce eggs, like the com- 

 mon round-worms and pin-worms. There is great diversity 

 in their habits and modes of development. Some of them 

 pass through alternate generations and remarkable trans- 

 formations, living in some cases, like Trichina, in two different 

 animals in the larval and adult states, in other cases pass- 

 ing the young states in the water, as free worms, and after- 

 wards becoming parasitic, when swallowed by the proper ani- 

 mal, in which alone they develop their reproductive organs 



