PARASITES 



361 



separating it from the form found in man. However, others feel that the 

 evidences are not sufficient for separating the rodent form and the human 

 form into two distinct species and apply the earlier name Hymenolepis nana 

 (von Siebold, 1852) to the forms found in mouse, rat, and man. Mor- 

 phologically the forms are identical and their life cycles 

 are the same. Also, the rodent form and the human 

 form are interchangeable, although in some experiments 

 they have not developed as readily in the alternate 

 host as they did in the host in which the parents 

 developed. However, Shorb (35) has shown a difference 

 between the rat form and that found in the mouse. He 

 has found that although strains from wild rats are 

 equally infective for rats and mice, strains from mice are 

 more infective for mice than for rats. Thus, it would 

 seem that while the parasites probably originated from 

 one form, which Augustine considers to have been that 

 of the mouse (20), there have since developed definite 

 differences in the three forms. 



The adult worm is quite small (Fig. 142). Measure- 

 ments given by Augustine (20) are: length 10 mm. to 

 45 mm. ; breadth 0.5 mm. to 0.7 mm. ; diameter of scolex 

 0.25 mm.; and length of hooks 14/xto 18 ju. The scolex 

 is globular, and on the rostellum the hooks form a single 

 row. They number from twenty -four to thirty. The 

 strobila may contain as many as 200 proglottids. The 

 eggs (Fig. 143A), which usually occur in large numbers in 

 the feces of infected animals, measure from 40 fxtotofx 

 in diameter. There are two membranes, the inner of 

 which gives rise to filiform projections at each pole. 



No intermediate host is required for development. 

 Grassi (1887) proved that infection occurred by the 

 ingestion of the eggs which are infective immediately 

 after they have passed out of the host. Upon entering the intestine 

 the eggs give rise to cysticerci which develop in the villi of the small 

 intestine, usually limiting themselves to the anterior one-half (Fig. 144). 

 Later the cysticerci produce adults which become attached to the epithelium 

 toward the posterior part of the small intestine. 



Although probably quite unusual, an intermediate host may be employed 

 in the life cycle. Bacigalupo (3) has shown that when eggs are ingested 



Fig. 142. — Adult 

 dwarf tapeworm, 

 Hy menolcpis nana . 

 (Enlarged.) {From 

 Stiles and Crane, 

 after Lcuckart.) 



