NEMATODA PARASITISM I 6 I 



a second animal : examples, Ollulanus, from Mouse to Cat ; Cucul- 

 lanus elegans, from Cyclops to Perch ; Spiroptera obtusa, from 

 Meal-worm to Mouse. 



12. The sexual form lives for a short time in the intestine of 

 a Vertebrate, and produces larvae which bore through the intes- 

 tinal wall and become encapsuled in the tissues : example, 

 Trichina spiralis. 



1 3. The sexual animal lives in the trachea of birds ; the ova 

 containing embryos are coughed up and are taken into other 

 birds with food. They quit the egg-shell and wander into the 

 air-sacs, and finally into the trachea : example, Syngamus. 



14. There are two larval forms; the first lives in water, the 

 second in the lungs of Amphibia, whence they wander into the 

 intestine and become sexually mature : example, Nematoxys longi- 

 cauda in Triton alpestris. 



Parasitism. 



1. Effect of Parasitism on the Parasite. The usual effect 

 of parasitism on the parasitic organism is that the various organs 

 necessary for a free life tend to degenerate, w T hilst there is a 

 multiplication and development of organs of adhesion, by means 

 of which the parasite maintains its hold on its host. There is 

 further an immense increase in the powers of reproduction, which 

 may take the form of an increase in the number of fertilised eggs 

 produced, or the parasite may at some time of its life reproduce 

 asexually, by budding, or fission, or parthenogetically. 



Of the various classes of animals which are more or less para- 

 sitic, the Nematodes show less difference between the free-living 

 and parasitic members of the group than obtains in any other 

 class. With few exceptions, such as Sphaerularia, Allantonema, 

 and one or two others, the parasitic forms have undergone but 

 little degeneration. It is true that they have no eyes such as 

 the free forms often possess, but in other respects, such as in the 

 nervous, muscular, and digestive systems, they do not show any 

 marked retrogression ; further, the mouth-armature is developed 

 in many free forms, and is not confined to the parasites. 



The group has developed no methods of asexual reproduction 

 by budding or fission, such as are found in Platyhelminthes ; and 

 the cases of an alternation of generations in which a sexual form 

 alternates with a parthenogenetic form, are rare, e.g. Rhab- 



VOL. II M 



