56 ILLINOIS BIOLOGICAL MONOGRAPHS [502 



The oral sucker has an average diameter in mounted specimens 

 of 0.040 mm. and the spherical acetabulum which is three-fifths of the 

 distance from the anterior to the posterior end is 0.025 mm. wide. 



In the dorsal wall of the oral sucker is set a stylet like that of Cer- 

 caria isocotylea. It is 0.028 to 0.03 mm. in length and thickened as in 

 the other species. 



The whole surface of the body is set with tiny spines contained 

 entirely within the cuticula and not set in rows; these thin out some- 

 what in the postacetabular region. 



Two groups of voluminous ducts pass up dorsad to the oral sucker 

 from the stylet glands, which fill most of the space from the acetabulum 

 to the pharynx. The diameter of one of these ducts is from 0.004 mm. 

 to 0.005 mm. The stylet glands are divided into two groups of from 

 ten to twelve in a group, and the individual glands vary in length from 

 0.017 to 0.022 mm., and in width from 0.010 mm. to 0.014 mm. 



Cystogenous glands are present both on the dorsal and ventral sides 

 of the body from the pharynx to the posterior end. 



In Cercaria polyadena the oral cavity is followed by a short pre- 

 pharynx 0.015 mm. in diameter. No traces were seen either of the 

 esophagus or the intestinal ceca. 



The excretory bladder is bicornuate and the excretory pore is 

 located dorsally at the base of the tail. The two horns of the vesicle 

 extend up to the posterior lateral margins of the acetabulum, and each 

 receives a small vessel. This receives a long vessel from the region of 

 the pharynx and a short vessel from the posterior end. The lining 

 of the vesicle is formed of a layer of slightly flattened cuboidal epi- 

 thelial cells with prominent nuclei and well defined cell boundaries. 



The anlage of the reproductive organs is not differentiated into its 

 individual parts. It is represented by an elongate s-shaped mass of nuclei 

 lying dorsad to the acetabulum and extending backward beyond its 

 posterior margin. 



Cercaria polyadena and Cercaria isocotylea are very much alike and 

 form the nucleus of a group the members of which present such uni- 

 formity of characters that they must be considered to be related. The 

 name Polyadenous cercariae may then be proposed as the name of a 

 natural group the members of which correspond closely to Cercaria poly- 

 adena. 



The following are the characters of the Polyadena cercariae : 



1. Development in gastropods in elongate sac-shaped sporocysts. 



2. Tail slender and less than the body length except when very 

 much extended. 



