80 WORM PARASITES. 



the former develop direct, whilst the latter (except 

 Boihriocephalus) can only develop by means of a 

 second host. Lastly, we know that no intermediate 

 host is necessary, as it has been shown that deve- 

 lopment can take place by direct transmission of 



ova. 



(2) CobholcVs supposed intermediate Host an Insect 

 or Mollusc. — Cobbold, after alluding to the permanent 

 unition of the two sexes, says : — " Clearly the eggs 

 can only escape by an eventual breaking-up of the 

 body of the parent ; ..... by whatever mode the 

 young escape the shell, it is clear that they are 

 already sufficiently developed to undertake an active 

 mi oration. A change of hosts is probably necessary, 

 but in the first instance they either enter the sub- 

 stance of fungi or other vegetable matters, or they 

 bury themselves in the soil a short distance from the 

 surface." Later Dr. Cobbold, in a communication 

 to the Linnean Society, suggests that the inter- 

 mediate habitation might be the larvae of certain 

 insects or land-mollusca. 



That they may enter the soil we know ; but unfor- 

 tunately no evidence has been brought forward of 

 sufficient importance in favour of either larval insects 

 or mollusca being the connecting-links in their life- 

 history. 



(3) Thirdly, Br. Walker s Theory must be referred 

 to. It is that the eggs falling upon the ground, and 

 also the embryonic Si/ngami, enter the Earthworm, 

 and there they live and undergo certain changes 



