V] Ttt2!kA'TOtfA 101 



arrangement in the Polyclada, has probably been the result of 

 laying the eggs in numbers surrounded by a capsule. This led to 

 a struggle amongst the eggs, resulting in the sacrifice of the smaller 

 to the needs of the larger ova, and this to the production of weak 

 ova to serve as food for the others, with the consequent differentia- 

 tion of the ovary into geruiarium and vitellarium. 



Class II. TREMATODA. 



The remaining two groups of Platyhelminths have taken to a 

 parasitic mode of life and this has to a great extent influenced 

 their organisation. The term parasite is applied to an animal 

 which lives on the external surface of or in the interior of another 

 animal, termed the host, and nourishes itself at the host's expense. 

 If the infesting organism like the Algae in Convoluta is made 

 use of by the host, the relation between the two is designated 

 symbiosis. If however the infesting animal only makes use of its 

 host's body as shelter and nourishes itself from the fragments of 

 the host's food, the relation between the two is referred to as com- 

 mensalism. The Trematodes have lost the external ciliation of the 

 skin, the ectoderm being everywhere covered with cuticle. In other 

 features of their anatomy they present a great resemblance to the 

 Triclade Turbellaria, and one family, the TEMNOCEPHALIDAE, may be 

 described as intermediate between the two classes of Platyhelminths, 

 since its members still retain patches of ciliated skin. For the most 

 part they live on or in the bodies of Vertebrates, attaching them- 

 selves either to the skin or to the alimentary canal or its outgrowths. 



One of the most characteristic features of Trernatoda is to be 

 found in the suckers by which they adhere to their prey. Often 

 indeed the lips of the mouth are thickened and muscular so as to 

 constitute an oral sucker, but there is always a ventral adhesive 

 disc provided with suckers or hooks or both. The mouth is 

 situated at or near the anterior end of the body ; it leads into an 

 oral funnel opening into a muscular pharynx, which by alternate 

 expansion and contraction pumps in the juices of the prey. Its 

 action is thus different from that of the pharynx of Turbellaria, 

 which, as we have seen, can act as a protrusible sucker. Behind 

 the pharynx the alimentary canal divides into two parallel forks 

 running back to the posterior end of the body, and beset with 

 branches which in some cases may unite with one another across 

 the middle line. It thus resembles what the alimentary canal of a 

 Triclade would become were the mouth shifted to the anterior end 



