106 PLATYHELMINTHES [CH. 



Class III. CESTODA. 



The Cestoda are sharply distinguished from the two preceding 

 classes of Platyhelminthes by the total absence of an alimentary 

 canal. They are all internal parasites, living in the alimentary 

 canal of their hosts, and absorb the half-digested juices of their 

 hosts through all parts of their skin. 



As in Trematoda, there is a well-marked cuticle which protects 

 the animal from the action of the digestive juice of the host. 

 The ectoderm has undergone an extraordinary modification. Its cells 

 have become long and filamentous, since they have acquired long 

 narrow necks, and the body of the cell with the nucleus has been 

 pushed downwards into the subjacent tissues. These necks are 

 however of very various lengths, and so the ectoderm, although 

 fundamentally a single layer of cells, presents the appearance of 

 a thick band of many layers of nuclei. The ectoderm cells inter- 

 mixed with longitudinal muscles form the outer or as it is termed 

 the cortical zone of the animal. Inside this and surrounded by the 

 circular muscles is the central mass of parenchyma, which is termed 

 the medullary zone (Lat. medulla, pith) in which the genital organs 

 and the excretory and nervous systems are embedded. Many of 

 the cells of the parenchyma secrete calcareous matter and form 

 the so-called calcareous corpuscles. 



Each portion of the body of a Cestode which contains a set of 

 reproductive organs is called a proglottis, and is budded off from 

 the anterior portion which is called the head 1 . The latter is pro- 

 vided with suckers, and generally, in addition, with a circle of hooks 

 situated on a prominence called the rostellum (1, Fig. 45, B). 

 "With the head the Cestode adheres to its host; its hinder part 

 or neck buds off proglottides in which new sets of reproductive 

 organs are formed, and this process is repeated an indefinite number 

 of times so that a chain of proglottides is formed. The oldest 

 proglottis is thus the hindermost. This method of segmentation 

 is called strobilisation on account of its resemblance to the 

 formation of the Ephyrae by a Hydra-tuba (p. 78) ; it differs from 

 the segmentation of the Annelida, where the new segments arise 

 not in the neck but in the tail. 



The excretory system consists of a larger and a smaller trunk 

 on each side, which unite in the last proglottis to end in a 



1 Some authorities regard the so-called "head" as the most posterior portion 

 of the animal. 



