54 ILLINOIS BIOLOGICAL MONOGRAPHS [500 



in contact with a surface and extended its body, the tail resumed its 

 iashings and swimming was started again with the body extended. This 

 was simply preliminary to the contraction of the body and the resump- 

 tion of the usual swimming position. 



The oral sucker of Cercaria isocotylea (Fig. 68, os) has an average 

 length in mounted sections of 0.04 mm. and a width of 0.037 mm., while 

 the acetabulum which is spherical in the living animal, is about two- 

 thirds of the distance from the anterior to the posterior end and has a 

 diameter of 0.036 mm. 



Set in the dorsal wall of the oral sucker is a large sylet (Fig. 67) 

 which is sharp pointed and has a thickening two-thirds of the distance 

 from its base to its tip. It is flattened ventrally, has a length of from 

 0.028 mm. to 0.030 mm. and a width at its base one-sixth of its length. 



The whole surface of the body back to the anterior margin of the 

 acetabulum is thickly set with tiny cuticular spines which are 0.003 

 mm. to 0.004 mm. in length. Back of this region the cuticula is entirely 

 smooth. 



On each side of the stylet open the ducts of the stylet glands, which 

 form a group on each side. The glands are elongate, sac-shaped, uni- 

 cellular and faintly granular, with a length of from 0.02 to 0.025 mm. 

 and a width of from 0.011 to 0.013 mm. They form two clumps of from 

 six to eight in a clump, in the region just in front and to each side of 

 the oral sucker. 



In this stage of development of the cercariae no cystogenous glands 

 are present. 



Except for the mouth, oral cavity, very short prepharynx, and 

 pharynx the digestive system of Cercaria isocotylea is undeveloped. The 

 mouth is very small having a transverse diameter of 0.012 mm. and the 

 round pharynx is 0.016 mm. in width. 



The excretory pore opens on the dorsal side just at the base of the 

 tail. The bladder is bicornuate consisting of a median part and two 

 lateral horns which reach on each side up to the middle of the acetabulum. 

 From the anterior and posterior regions on each side can be traced small 

 vessels which unite into a short common duct to open at the tips of the 

 horns. (Fig. 68, ex) 



The anlage of the reproductive organs is not divided into definite 

 parts. It consists of a mass of small nuclei which lies dorsad and just 

 in front of the anterior margin of the acetabulum. This connects with 

 a larger mass which is dorsal to the posterior part of the acetabulum by 

 a broad band running around dorsal to the left margin of the sucker. 



Among the specimens of Lymnaea reflexa from Chicago was one 

 which contained a number of elongated unpigmented sporocysts, which 



