PLATYHELMINTHES 



191 



found in the Turbellaria. As in the Turbellaria, the gut may have 

 lateral diverticula which branch freely. The Trematoda have, how- 

 ever, lost the external ciliation of the Turbellaria (Fig. 151). The 

 ectoderm is represented by cells sunk into the parenchyma in much 

 the same way as nuclei of the ectodermal cells in the pharynx of the 

 Tricladida. But the outer portion of the cell is lost in the Trematoda 

 and its place is taken by a thick cuticle, which is often armed with 

 spines. Suckers are always present for attachment to the host and 

 are of large size. The presence of these suckers and their shape makes 

 it possible to divide the Trematoda proper into two orders : (i) Hetero- 

 cotylea, (ii) Malacocotylea. 



^^■jten. 



Fig. 150. 



Temnocephala minor y X12. After Haswell. g.o. genital opening; 

 M. mouth; sue. sucker; ten. tentacles. 



Order HETEROCOTYLEA 



In the Heterocotylea there is a large posterior sucker stiffened with 

 chitinous supports. It is often subdivided, as in Octobothrium or 

 Polystomum (Fig. 152). In the Malacocotylea the sucker is not always 

 posterior, it often moves forward on the ventral surface so that, as in 

 Fasciola, it comes to lie one-third of the body-length from the anterior 

 end. It is never provided with chitinous supports. All the Hetero- 

 cotylea are ectoparasites with the single exception of Polystomum which 

 occurs in the bladder of the common frog, of which from 3-10 per 

 cent, are infected by it. They are confined to one host only. The 

 Malacocotylea are all internal parasites and pass from one host to 



