190 THE INVERTEBRATA 



a cosmopolitan tropical form that often turns up in greenhouses. It 

 is often a foot long and is easily recognized by the axe-shaped head. 

 Rhynchodemus terrestris, a small form 6-8 mm. long, is a British 

 representative of this division. It is found in damp situations under 

 the bark of decaying trees and fallen timber. 



Order POLYCLADIDA 



These are entirely marine. The gut has many diverticula leading out 

 from a not very conspicuous main stem. The mouth has shifted to the 

 posterior end. The germarium and the vitellarium are combined into 

 one organ but the different portions may be distinguished in sections 

 by suitable staining. There are separate male and female openings. 

 Some members of this group attain a considerable size, six inches or 

 more in length. A small sucker is found in some forms behind the 

 genital pore. ThysanozooUy a member of this order, has the dorsal 

 surface covered with papillae into which run coeca from the intestine. 

 In Yungia there are similar papillae also containing diverticula of the 

 gut, some of which open to the exterior. 



Class TREMATODA 



The Trematoda may be defined as Platyhelminthes which are para- 

 sitic (or, in Temnocephalea, epizoic) ; which retain the enteron ; which 

 in the adult have outside the ectoderm a thick cuticle ; which have 

 suckers; usually, but not always, a sucker on the ventral surface in 

 addition to one surrounding the mouth ; ventral sucker is subdivided 

 in some forms. 



The Trematoda are linked to the Turbellaria by the little group of 

 animals which constitutes the order Temnocephalea containing the 

 genus Temnocephala and one or two others. These animals have a very 

 discontinuous distribution and live attached to the surface of fresh- 

 water animals, chiefly Crustacea. They do not feed on their host but 

 use it as a resting place from which they catch rotifers, Cyclops, and 

 other small water animals for food. The five tentacles at the anterior 

 end makes the group easily recognizable (Fig. 150). The epidermis 

 is retained as a nucleated syncytium which secretes outside it a thick 

 cuticle. In the region of the tentacles rhabdites occur. The mouth is 

 anterior, the gut has the same shape as in the Rhabdocoela. There is 

 a large sucker at the posterior end with the common male and female 

 opening in front of it. The nervous system is of the primitive network 

 type. 



The rest of the Trematoda are all parasitic but they resemble in 

 general shape the Turbellaria. They have retained the mouth, which 

 is anteriorly placed, and the gut, which, however, is bifid, a shape not 



