306 SOME COMMON INTERNAL PARASITES [CH. IX 



ohe end, a small ciliated larva about 015 mm. long and 

 0'04 mm. broad. The larva is provided with a double 

 eye-spot, and at its anterior end has a central papilla which 

 can be elongated to form a finely pointed awl. By means 

 of its cilia the creature swims in the water and searches 

 for a small water-snail, Limncea truncatida. In our own 

 country a marked preference is shown for this particular 

 species of gastropod, though other species may be also 

 attacked. Elsewhere however the embryo has accom- 

 modated itself to faunal differences. For example, Lutz 1 

 states that in the Hawaiian Islands, where there have been 

 severe outbreaks of " rot," the intermediate host is Limnasa 

 cahuensis. If the larva fails to find a snail within about 

 eight hours it dies : on the other hand, if successful in the 

 quest it proceeds at once to bore a passage into the soft 

 tissues of the mollusc. This it achieves by elongating its 

 awl and spinning round about its long axis by means of its 

 cilia : it becomes for the nonce an animated drill. Mere 

 penetration into the snail is not necessarily crowned with 

 future success. In the tougher, more muscular portions of 

 the body the larvae soon perish. But in the pulmonary 

 chamber and other highly vascular situations the larva, 

 having no further need of organs of active locomotion, 

 throws off its ciliated covering and becomes a mere sac. 

 At the same time the eye-spots, being no longer of service, 

 separate and eventually disappear. In this stage the 

 organism is known as the sporocyst Under favourable 

 conditions it grows rapidly and in two or three weeks 

 attains a length of about 0*6 mm. 



1 Centrabl f. Bakter. u. Parasitenk. xi. 1892. 



