NO. 1788. NORTH AMERICAN ERGASILID^— WILSON. 333 



single spine. The arrangement of spines and setse on the first four 

 pairs of legs is as follows: First exopod, I-O; 0-1; II-5: endopod, 

 0-1; 0-1; II-4: second exopod, I-O; 0-1; 0-6: endopod, 0-1; 0-2; 

 1-4: third exopod, I-O; 0-1; 0-6: endopod, I-O; 0-2; 0-5: fourth 

 exopod, 0-0; I-O; 0-4: endopod, 0-1; 0-2; 0-4. 



The anatomy of the muscular, digestive, and reproductive organs 

 of this species have been described on pages 285 to 302; the nauplius 

 and two metanauplius stages of development were described and 

 figured under the ontogeny of the Ergasihnae, pages 319 to 326. 



Body a clear cartilage color, translucent in young females, but be- 

 coming dense and opaque in the adults; ovaries and testes an opaque 

 white. Eggs also an opaque white when freshly extruded, acquiring 

 blue pigment gradually upon development, until when ready to hatch 

 the entire egg-sacks appear blue to the naked eye. 



Total length, 0.8 to 0.9 mm. Length of cephalo thorax in adult, 

 0.66 mm. Width of same, 0.58 mm. Length of egg sacks, 0.9 mm. 

 In young females with their first egg-sacks the body is relatively longer 

 and narrower, and the proportion between the length of the cephalo- 

 thorax and the remainder of the body is as 11 to 8 (see fig. 21). 



(centrarcliidarum, of or belonging to the Centrarchidse, the family 

 of fish upon whose gills this parasite is found.) 



The red eye (Amhloplites rupestris) is the most common host of this 

 parasite, and nearly every one of them which the author has examined 

 has been found infested to some extent. During the summer of 1906 

 quite a number of red eyes were examined at Lake Maxinkuckee, 

 Indiana, for the purpose of ascertaining their food and the kinds 

 of parasites which infested them. These were all young fish, 2 inches 

 long and upward, in the second year of their growth. Almost every 

 fish showed some of this species on its gills, and several of them, 

 scarcely 3 inches long, yielded from 15 to 25 of the parasites apiece. 

 An adult red eye often has as many as 75 or 100 of these creatures on 

 its gills. The National Museum collection includes four bottles of 

 specimens from this host, taken at Lake Maxinkuckee and numbered, 

 respectively, 38609, 38610, 38620, and 38632; one bottle containing 

 a single female taken from the gill of a pike perch, Stizostedion 

 vitreum, and numbered 38616; another single female found on the 

 gill of a blue-gill, Pomoxus sparoides, numbered 38634; 15 females 

 taken from the gills of the small-mouth black bass, Micropterus 

 dolomieu, numbered 38624; 3 females from the gills of the war-mouth 

 bass, CJisenohryttus gulosus, taken in Lost Lake, close to Lake Maxin- 

 kuckee, and half a dozen females from red eyes and blue-gills, taken 

 at Lake Winona, Indiana, these last two being numbered, respec- 

 tively, 38613 and 38630. 



