NO. 1788. -NORTH AMERICA-^ ERGASILID.E— WILSON. 349 



Only these two mouth-parts are shown, and nothing is said of any 

 others in the text. In view of such mistakes, we shall have to wait 

 for further testimony before deciding as to the validity of the species. 

 But this need not hinder us from locating the genus. The present 

 author believes TJiersitina to be a valid genus and distinct from 

 Ergasilus, for the following reasons: 



1. Its habitat. Ergasilus species are always found clasping the 

 gill filaments with their second antennae; Thersitina is found with 

 its second antennae buried in the skin on the inside of the operculum. 



2. Its body form. In mature females of Ergasilus the cephalo- 

 thorax is sometimes swollen by the development of the ovaries and 

 oviducts and their contents, and becomes more or less cylindrical; 

 but it never approaches a sphere, and the second thorax segment 

 does not share in the inflation. 



3. The structure and attachment of the antennae. The first 

 antennae are very short and five-jointed in Thersitina, while in 

 Ergasilus they are relatively longer and six-jointed. The second 

 antennae in Thersitina are short and stout, and closely resemble the 

 second maxillipeds in the Caligidae. In Ergasilus they are long and 

 slender and bear no resemblance to those maxillipeds. 



4. The structure and number of the mouth-parts. In Ergasilus 

 females the maxillipeds are entirely lacking; in Thersitina they are 

 present behind the other mouth-parts and of peculiar structure. 

 T. Scott is a very careful and accurate observer, and one who has had 

 much experience with both parasitic and free-swimming forms. His 

 description, therefore, is entitled to great confidence, especially in 

 view of the fact that the other observers have not given much atten- 

 tion to these maxillipeds. Hence we may concede the validity of 

 the genus and present the following 



GENUS DIAGNOSIS. 



Head fused with the first thorax segment ; resultant cephalothorax 

 and the first free segment inflated into an ellipsoid or sphere, to the 

 ventral surface of which at the posterior end is attached the com- 

 paratively minute remainder of the body; other free segments less 

 than a quarter of the diameter of the cephalothorax, diminishing 

 regularly in size. Genital segment not much enlarged; abdomen 

 three-jointed, joints about equal. 



Anterior antennae small, scarcely reaching halfway to the margin 

 of the carapace, five-jointed and well armed with setae. Second 

 antennae short and stout, suited for burying in the tissues of the host, 

 like maxillipeds, rather than for clasping. Mouth-parts consist- 

 ing of falciform mandibles, rudimentary first maxillae like those 

 of Ergasilus and Bomolochus, simple second maxillae, and a pair of 

 three- jointed maxillipeds behind the other parts. Egg-cases ellipsoi- 



