1 62 NEMATHELMINTHES CHAP. 



donema nigrovenosum ; and it seems possible that eveii when 

 parthenogenesis has been described, further observation may show 

 that the parthenogenetic stage is really a protandrous hermaphro- 

 dite, in which case the alternation of generations in Nematodes, 

 i.e. the hermaphrodite alternating with the dioecious form, is a case 

 of heterogamy or the alternation of two sexual generations. 



On the other hand, parasitic Nematodes produce enormous 

 D umbers of eggs. Van Beneden states that 60,000,000 have been 

 computed in a single Nematode, and this multiplication of ova is 

 absolutely necessary, for the chance of the embryo reaching the 

 right host, in which alone it can develop, is always a small one. 



It is a common thing to find that parasites are either herma- 

 phrodite or that the male is degenerate, as is the case with many 

 of the parasitic Crustacea, but with one or two exceptions the 

 Nematoda are bisexual, and although, as a rule, the males are 

 smaller than the females, they show no other trace of degeneracy. 



In spite of the fact that the class as a whole shows but few 

 special modifications consequent on a parasitic mode of life, it is 

 clear that the Nematoda are peculiarly adapted for such a mode of 

 life. Their elongated thread-like bodies afford little resistance to 

 the passage of the food, which, as it passes through the intestine 

 of the host, might tend to carry the parasites out of the body. 

 At the same time their shape enables them to pierce and wriggle 

 through the various tissues without making any very serious 

 lesions such as might prove fatal to their host. Their extra- 

 ordinary power of resisting desiccation both in the egg and in 

 the adult state vastly increases their chances of ultimately hit- 

 ting on the right host. They are capable of living in a state of 

 suspended animation for months, and even years when dried 

 (vide p. 136), and of resuming their activity on being moistened. 



The great faculty this group shows for living parasitically is 

 evinced by the extraordinary variety of life-history presented by the 

 different species. There is scarcely a stage which may not be para- 

 sitic ; the eggs, the larvae, the adults are all in some cases free, in 

 others parasitic, and in many cases first the one and then the other. 



2. Occurrence and Effect of the Parasite on the Host. Vou 

 Linstow states that the only law that can be derived inductively 

 from the study of the life-history of Nematodes is that those that 

 live in animals never pass through all their stages of development 

 in the same organ ; consequently, in considering the distribution of 



