TRANSMIGRATIONS AND METAMORPHOSES. 283 



varying in length from a few millimetres to forty or 

 fifty centimetres. 



They are not all parasites, as has been thought, 

 since some are found in the sea, and others in damp 

 earth, in putrid matter, and even on plants and their 

 seeds. The migrations of nematodes are subjects of 

 great interest. Their changes of form are usually not 

 very considerable ; but the modifications in their sexual 

 apparatus, whether in the same individual, or in the 

 succeeding generations, are very cm'ious. 



When we consider the numerous encysted and 

 agamous nematodes, which are found in the different 

 orders of mammalia, birds, reptiles, batracliians, and 

 fishes, there is little doubt that all these beings are only 

 migratory parasites, which pass together with their hosts 

 into the animal to which they are destined. They are 

 found, like ascarides, in animals of all classes. Some 

 are to be met with in all the organs— the brain, the eye, 

 the muscles, the heart, the lungs, the tracheal artery, 

 the frontal sinus, the digestive tube, the skin, and even 

 in the blood. Sometimes the two sexes live under the 

 same conditions ; sometimes the male is dependent on 

 its female, or else one generation is parasitical, and 

 the next is independent. There is a great diversity with 

 respect to development. Some nematodes, like trichinae, 

 are developed so rapidly, that the embryos are abeady 

 perfect in the egg before it has quitted its mother. 

 Others, Uke the ascarides lumbricoides, lay eggs, in 

 which the embryos do not appear till several weeks or 

 many months after they have been laid. Between these 

 two extremes we find all the intermediate degrees. 



Diezing, who has done more for systematic helmin- 



