Fig (IX)
7.
Ova of Davainea urogalli in different stages of development. It grouse, this species (Fig. 6) is by far the largest and by far the most common. exhibits little seasonal variation, and is found in considerable numbers all the year round. The birds become infected at an early age. We have taken one specimen of this cestode, 35 cm. in length, from the intestine of a bird about three weeks old. D. urogalli is found in the small intestine. There are sometimes part of it found in other portions of the alimentary canal.
As a rule, three or four are met with. At other times, especially in weakly birds, there will be dozens (Fig. 6), and these fill up the lumen of the intestine to such an extent that it is difficult to see how food can pass along.
D. urogalli, like most cestodes, produces a very great number of eggs (Fig. 7). At any one time there may be, at a rough estimate, at least 100,000 ova in the body of the tape-worm; but this figure is no measure of the reproductivity of the cestode, because as fast as new segments break off at one end, new ones are formed in the head, and the animal goes on producing new segments like a recurring decimal. Hence the eggs of this cestode must be scattered in count- [GROUSE flies and moths, some young hemipterous insects, and the pupa of two true flies.
In searching for the larval or cysticercus stage of these and the other cestodes, we have examined a considerable number of insects which occur commonly on grouse moors. We have also carefully searched the bodies of many fresh-water crustacea which abound in the pools and tarns from which grouse drink. But hitherto our searches have met with no success. One specimen of cestode which infests the common fowl is said to have its second host in several species of the slug Limax, but we have not succeeded in finding cysts of either form of tape-worm in this slug. 10. Davainea cesticilla (Molin). - A second much smaller species of Davainea occurred but twice in the many hundreds of grouse we have investigated. It is a common parasite of turkeys and chickens, but its occurrence is so rare amongst grouse that it may be neglected as the cause of disease.
II. Hymenolepis microps (Diesing).- This third species of cestode lives in the duodenum, though after death the individuals may wander into other parts. It is an extremely delicate, fine, transparent tapeworm, and it exists at times in enormous