2l6 THE INVERTEBRATA 



Polycladida the entry of the ovum by the sperm takes place after the 

 extrusion of the polar bodies, whereas in other Turbellaria this 

 follows the entry of the sperm. These facts have inclined modern 

 authorities to the belief that the Polycladida are more nearly related 

 to the primitive Acoela than to the Rhabdocoelida and Tricladida. 



A further point of interest in this group is that development is not 

 direct. It leads to the production of a larva, known as "Miiller's 

 larva" (see Fig. 155), which is characterized by projecting processes 

 and a band of ciha. As we have seen (p. 145), projecting processes 

 (arms) and bands of cilia are characteristic of the larvae of many 

 forms belonging to several phyla ; but their presence is probably an 

 adaptive feature and it is unwise to base phylogenetic speculations 

 on them. "Miiller's larva" is a planktonic, and therefore a dis- 

 tributive stage, in the life history. At metamorphosis, when the 

 animal adopts the crawling progression of the adult, the larva loses 

 the projecting arms and the bands of cilia, while at the same time it 

 loses its rotundity, becoming flattened and elongated. 



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. Thysanozoon, a member of this order, has the dorsal 

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

 In Yimgia 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; the ventral sucker is sub- 

 divided in some forms and may also be stiffened with a ringlike 

 chitinous skeleton. 



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 possession of five tentacles at the 

 anterior end makes the group easily recognizable (Fig. 156). The epi- 

 dermis 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 



