188 ILLINOIS BIOLOGICAL MONOGRAPHS [188 



CHOANOSCOLEX ABSCISUS (Riggenbach 1896) La Rue 



[Fig. 151] 



1896 : Ichthyotaenia abscisa Riggenbach 1896 :193-210 



1911 : Choanoscolex abscisus La Rue 1911 :479 



Specific Diagnosis : Characters of genus. Worms small, 25-30 mm. 

 long, breadth up to 0.765 mm. Number of proglottids as many as 70. 

 Head somewhat conical with a fold of tissue at the base at times partly- 

 covering the suckers. Apex of head flattened. Maximum breadth of 

 head 0.476 mm. No rostellum, no fifth sucker. Suckers large oval, 

 0.306 mm. long by 0.136 mm. broad. First proglottid longer than broad. 

 Tenth proglottid 0.476 mm. broad by 0.102 mm. long. Segments near 

 middle of worm broader than long or nearly quadrate. Last proglottid 

 1.02 mm. long by 0.765 mm. broad. Proglottid margins straight, pos- 

 terior corners rounded. Last ploglottid with bluntly rounded end. 



Sexual organs as in genus. Genital aperture situated at end of 

 anterior %-% of the proglottid. No genital papilla. Testes, about 100 

 in number, 0.054 by 0.045 mm. in size, arranged in a single layer be- 

 tween vitellaria. Coils of vas deferens voluminous. Coils of ductus 

 ejaculatorius few. Cirrus swollen, with large lumen. Cirrus-pouch 

 large, pear-shaped, muscular, reaching nearly to middle of segment. 

 Length of cirrus-pouch 0.34 mm. Vagina anterior or posterior to cirrus- 

 pouch. Lumen of first part of vagina large. Receptaculum seminis 

 present. Ovary bilobed, posterior, unbranched. Vitellaria lateral, fol- 

 licular. Uterus a median tube with many lateral outpocketings. Eggs, 

 0.021 mm. long by 0.016 mm. broad. 



Habitat: Silurus sp., Rio Paraguay, South America. 



This species was described by Riggenbach (1896) as a species of 

 Ichthyotaenia. La Rue (1911:479) established for it a new genus, 

 Choanoscolex and made this species the type of the new genus. 



The material was collected by Dr. Ternetz in Paraguay, 1894. The 

 following description is based on the work of Riggenbach (1896). Rig- 

 genbach considered this form to be a species of Ichthyotaenia and his 

 comparisons are made with species of that genus rather than with 

 Corallobothrium. 



The worm is small being 25-30 mm. in length. It is made up of 

 about 70 proglottids whose form varies according to the age. The scolex 

 (Fig. 151) varies from the usual shape in being somewhat conical in- 

 stead of spherical and in having at the base of the cone a fold of tissue 

 or a mantle which may be partially drawn over the suckers. The tip 

 of the cone is flattened. The maximum breadth of the head is 0.476 mm., 



