498 HARRY M. MILLER, JR., AND FLORA E. NORTHUP. 



SUMMARY. 



From the data resulting from the examination of almost nine 

 thousand specimens of Nassa obsoleta Say it seems clear that 

 there is a semi-annual rise and fall in the larval trematode infes- 

 tation. In view of the fact that none of the adults of these 

 larvae are known it is difficult to explain these phenomena. In 

 all probability migrations of the definitive hosts, and the degree 

 of their infestation, affect the seasonal fluctuations; and other 

 factors are probably the life span of Nassa, and the effect of 

 parasitism upon it. The relative importance of these factors is 

 not clear. 



BRIEF DESCRIPTION OF NEW SPECIES CITED IN THIS PAPER. 



CERCARIA SETIFEROIDES sp. nov. 

 (Figs. 1-3, 6, 10.) 



Trichocercous distome cercaria with opaque yellowish body and 

 characteristic excretory vesicle. Contractile body varying from 

 140 M to 268 n in length and 104 M to 156 M in width; average 

 length 187 fj. and width 126 /r, average tail length 486 /z- Oral 

 sucker slightly elongate, 64 M in average length and 34 n in width ; 

 ventral sucker smaller and oval, 40 /* in width and 34 /* in length. 

 Surface of body finely pebbled, and completely covered with 

 short, regularly arranged spines. Conspicuous pigmented eye 

 spots, composed of large spherical granules and a so-called lens. 

 No spines on tail, but on either side of it rows of long setae; 

 usually seven setae per group, the longest in the center; about 

 thirty pairs of setae groups arranged along sides of tail opposite 

 to each other. Well developed digestive system clearly seen in 

 both living and preserved material; mouth antero- ventral in 

 oral sucker; short prepharynx followed by large oval pharynx, 

 24 IJL in length and 16 M in width; short esophagus, and two wide 

 intestinal ceca extending to posterior end of body; jelly-like 

 contents of ceca with great affinity for intra-vitam neutral red, 

 in contrast to all other structures in body. Eleven pairs of 

 larval glands, preacetabular in position, arranged in three groups; 

 no observable ducts from the most posterior six glands; glands 

 of all groups strongly acidophilic in all combinations of stains; 



