64 ILLINOIS BIOLOGICAL MONOGRAPHS [64 



soon encyst in the free tissue of the host. The second generation redia is 

 aheady producing germ balls before it comes to take up a free existence out- 

 side the first generation redia. 



The internal systems of organs of Cercaria flabelliformis are of considerable 

 interest. However, since they are described in detail in the respective sections 

 in the morphological division of this paper (pp. 37, 45, 54) they will not be 

 treated here. 



Valuable data on holostomid anatomy are obtained by a comparison of 

 Cercaria flabelliformis with the following species: 



Tetracotyle pipientis nov. spec. 



This species of larval trematode was found in March 1917 in the mesentery 

 and pericardium of a large number of Rana pipiens collected in the vicinity 

 of Chicago, Illinois. All of the frogs were more or less infected with this 

 holostome. The infection consisted of creamy oval yellow cysts, either single 

 or in grape-like clumps. Each cyst consisted of many lamellae, and innermost, 

 a tough cyst membrane. The inner membrane stains a deep brown with 

 iodine in 70 per cent ethyl alcohol. The gross measurement of the cyst ranges 

 from 0.5 mm. to 0.76 mm. in lesser diameter and 0.7 mm. to 1.0 mm. in greater 

 diameter, while the inner membrane is about 0.3 mm. by 0.5 mm. Within the 

 inner membrane is the larva, tightly coiled at one end of the cavity, while 

 the remainder of the cyst, often two-thirds of the volume, is filled with accum- 

 ulations of large excretory granules. Some of these granules have fused to 

 form single clumps as large as the larva 



WTien the lamellae and cyst membrane are teased open and the larva is 

 allowed to work its way out, the body becomes expanded and flattened. It 

 then measures 0.5 mm. in length by 0.37 mm. in trans-section (Fig. 47). The 

 oral sucker is 75)U in diameter. It lies in an anterior cone of the body. Antero- 

 lateral prominences and the blunt posterior portion of the body give a lyrate 

 outline to the worm. The primitive genital pore, 80;u in diameter, lies in a 

 plane where the anterior and posterior portions of the body join, just within 

 the suctorial pocket. The free ventral wall of this pocket is often folded 

 backward so that it fits down snugly against the body. At other times it 

 bulges out so that the pocket cavity is a large ovoid atrium. The acetabulum 

 is represented by a single lappet situated behind the primitive genital pore. 

 The non-muscular accessory suctorial grooves consist of long narrow slits, 

 directed obliquely inward toward the acetabulum. The entire worm is covered 

 with minute anterior and posterior spines, equally prominent. The primitive 

 genital pore is crowned with a ring of fused spines. The lateral suctorial 

 organs are surrounded by a band of discrete spines imbedded in the tissues. 



The worms examined were all filled with excretory granules. A careful 

 study of the larva showed the main course of the excretory trunks to appear 

 as shown in figure 48. The median posterior excretory pore, sUghtly dorsal, 

 conmiunicates with the bladder which merges imperceptibly with the paired 



