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UNITED STATES NATIONAL MUSEUM BULLETIN 296 



need a secondary pair of cirri near the mouth to remove the food 

 material and transport it to the mouth. The intermediate step between 

 typical thoracicans and the acrothoracicans is exemplified by the genus 

 Ihla Leech 1825, which shows a slight separation between the first 

 pair of cirri and the remaining pairs, Ihla also has reduced plates, a 

 dioecious sexual condition with dwarfed males, and the placement of 

 the adductor muscle posterior to the esophagus (fig. 40). 



The most primitive family of the order Acrothoracica is the family 

 Lithoglyptidae. The members of the genus Weltneria resemble the 

 thoracicans in having biramous cirri, a mandibular palp, an anus, 

 and two ventral ganglia, as well as the largest number of cirri (six 



FLAGELLAR 



DENSE 



TUBULES 



B 

 PLANE 



Figure 45. — Diagram of cross section and longitudinal section of sperm of T. lateralis. 

 Letters correspond to cross section planes in figure 44. The longitudinal diagram is in 

 plane E of figure 44. 



