3l6 E. H. STRICKLAND. 



At first sight one would suppose that the retardation in develoj > 

 ment of the histoblasts in parasitized Simiilium larva? is simply 

 due' to a lack of proper nourishment for thr>e <>r-,m^, MIHV the 

 lar\-a, besides having to supply the requirements of its own 

 developing larval tissues, has also to supply the demands of its 

 fast-growing parasite. This may be true to a certain extent, 

 but later observations, when another parasite is also present, 

 indicate that there must be some other more potent factor \vhch 

 accounts for this inhibition. This second parasite is a Spotozoon 

 which, owing to the vast numbers in which it occurs in a single 

 host, is far more bulky than the worm and must, one would 

 imagine, make far greater demands on the resources of its host. 

 In this case, however, the histoblasts are usually unaffected in 

 size, though in many cases they are distinctly smaller than normal. 

 Two individuals, however, were seen in which the histoblasts 

 were minute. On dissection it was found that, in each case, a 

 small worm measuring only some 7 mm. was living embedded in 

 the mass of spores, and it was evident that this minute worm was 

 responsible for the retarded condition of the discs, even though 

 it had evidently absorbed very little nourishment. One must, 

 therefore, in all probability look to some toxin secreted by the 

 worm as the cause of the inhibition of development in the discs. 



The researches of Verson and Bolle, as quoted by Fischer 

 ('06), proved in the case of lepidopterous larva? that in their 

 early stages their body fluid is alkaline and that this alkalinity 

 decreases as the larva matures. This would suggest that an 

 alkaline condition encourages the growth <>1 larval tissues whereas 

 acidity, or the absence of alkalinity, permits of the development 

 of the adult organs. Hence the histoblasts would develop but 

 slowly till the larva? are nearing maturity when the decreased 

 alkalinity of the blood allows the "adult" enzymes to stimulate 

 the cells of these discs to rapid division. I have been unable 

 to find any account of the excretions of Mermis or of closely 

 related Nemathelminthes, but should they be proved to have an 

 alkaline reaction the probability of the above contention would be 

 very greatly strengthened, for here we should have a case of the 

 alkalinity of the system being maintained in maturing larva?, and 

 thus preventing the normal, though slow, development <>t the 



