~ 
as a means of defence against Parasites 55 
I may also point out that the body cavity in Arthropods is of a 
rather special nature, especially in such forms as the larvae of the 
Insects, for it extends uninterruptedly from one end of the body to 
the other, and the organs can be subjected to considerable displace- 
‘ment in it without injury. Certain parasites are thus able to 
develop until they have attained a size almost equal to that of 
their host. Finally, as Mr F. A. Potts has suggested to me, the 
fact that the body cavity is a haemocoele is important, for parasites 
which develop in a haemocoele enjoy considerable advantages from 
the point of view of respiration and nutrition. 
I therefore conclude that the very considerable parasitism to 
which Arthropods in general and Insects in particular are subject 
is not necessarily a proof of the inefficiency of the cuticula as a 
defensive structure. It appears to me more likely that this high 
rate of parasitism is due in part to the fact that a great many of 
the parasites are themselves Arthropods, in part to the Arthropod 
structure and life history which renders the members of this group 
especially able to support parasitic invasions. It seems highly 
probable that the cuticular armour, and the function of ecdysis 
correlated with it, in reality arrests a very considerable part of 
the violent attack which many members of the Arthropoda are 
obliged to sustain. 
