74 PLATYHELMINTHES 



tremendous lacerations sometimes, but the tissue destroyed is 

 easily regenerated. 



The cocoons contain about a score of eggs with several hundred 

 yolk cells containing food. The larva at a certain stage develops a 

 temporary larval mouth and gullet, and swallows the food yolk, 

 by which it is able to grow rapidly. The larval mouth disappears 

 and a new permanent mouth replaces it. The embryo is like its 

 parents when it leaves the shell. Adult over-nourished Planaria 

 undergo fission, the posterior portion quickly regenerating a head. 



The nervous systejn consists of the brain, a bilobed affair with two 

 longitudinal nerve cords or ventral nerve chains running backwards, 

 and giving off, internally and externally, transverse branches which 

 also subdivide. The inner ones frequently anastomose to form 

 commissures. The brain is rather diffuse and made up of groups of 

 ganglion cells, nerves, and has transverse fibers connecting the nerve 

 cords. The animals are responsive to all sorts of stimuli, and the 

 eye-spots, the lateral olfacto-gustatory organs, and the anterior end 

 are all well supplied with nerve endings. 



Food. — Planaria live upon small Crustacea, larvae of Crus- 

 tacea, water mites, and Rotifera, as well as on plant food such as 

 diatoms and algae. They are also said to attack earthworms. 



Class 2. Trematoda. — The Trematoda are leaf-like parasites 

 with no cilia in the adult, a thick cuticle, ventral suckers, sometimes 

 with posterior hooks, and with a forked or branched alimentary canal 

 ending in blind branches, the cecae. 



Type of Group. — The liver fluke, Fasciola {Distomum) hepatic a. 

 The adult liver fluke lives in the bile ducts of sheep, cattle, horses 

 and pigs, and sometimes occurs in man. It is soft-bodied, flatteneft 

 and leaf-like with a triangular lobe at the broader end, and with two 

 well-developed suckers, the anterior one being perforated by the 

 mouth, and the posterior one ventrally situated. The disease 

 " liver rot," which is especially prevalent among sheep pastured on 

 snail-inhabited marshy ground, has killed many millions of sheep. 



Life History. — The eggs of the liver fluke are 1/180 of an inch 

 long, with brownish shells, having a greenish sheen. They can 

 develop only in water where they hatch in from four to five weeks. 



The larva or miracidium is 1/125 of an inch long, ciliated, and 

 with a single eye, but has no gut. If the larva does not find a 

 snail of the right species in from eight to ten hours, it dies. Having 

 entered the lungs of the snail, it loses its cilia, becomes broader and, 



