NO. 1788. NORTH AMERICAN ERGASILID^— WILSON. 313 



ARTIFICIAL KEY TO THE GENERA. 



a. Body somewhat flattened, considerably wider than thick; second antennae slender 

 and nearly as long as the entire body; first and second thorax segments free from 



the head but fused together Macrobrachinus Hesse, 1871, p. 264. 



a. Body cylindrical or spherical; second antennte stout and no longer than the 



cephalothorax; first thorax segment fused with the head, the others free b. 



b. Carapace inflated into a sphere; first and second antennae about the same length, 

 short and stout; maxillipeds in the male with several strong apical spines, but 



without a claw Thersilina Norman, 1906, p. 347. 



b. Carapace cylindrical or flattened, much longer than wide; second antennae four 

 or five times as long as the first pair; maxillipeds in the male with a single ter- 

 minal claw and no spines Ergasilus Nordmann, 1832, p . 326. 



ONTOGENY. 



Historical. — Nordmann, who founded this family, gave also a figure 

 of the newly hatched nauplius of Ergasilus sieboldii, his type-species. 

 In his text he presents an account of the development of the genus 

 and a description of the nauplius, which were remarkably good when 

 we consider the date of his work (1S32) and the fact that he was deal- 

 ing with a family previously unknown. His account, however, 

 stopped with the nauplius and included none of the stages which 

 intervene between that form and the fully developed adult. 



Subsequent investigators have added practically nothing to our 

 knowledge of this development. One or two of them, like Hesse and 

 Beneden, have given us figures of the nauplius larvae of other species. 

 But these have not been supplemented by any text description, and 

 the figures themselves have been far too small to give the details of 

 structure. 



And so the matter rested until 1904, when Hofer published his 

 Handbuch der Fischkrankheiten, in which he embodies the sub- 

 stance of Nordmann' s account and adds many statements of his own. 

 Ho gives us no idea how the facts presented in these statements were 

 ascertained, and unfortunately the most of them are at least ques- 

 tionable. For instance, he says (p. 8) "Die jungen Tiere, welche die 

 Naupliusgestalt aller Copepoden haben, brauchen ca 1 Woche bis 

 zum Ausschliipfen." 



If he means by this that it takes the eggs a week to ripen and hatch 



after they are extruded into the external sacs, he is probably wrong, 



for it requires a much longer period in every species observed by the 



present author. 



If, on the other hand, he means that after the eggs are fully ripe 



and have begun to hatch it requires a week for them all to get out 

 of the envelopes, he is undoubtedly again mistaken, for all the eggs 

 in both strings have repeatedly hatched out under observation 

 within a few hours. 



Furthermore, his statement that these nauplii "suchen sich sofort 

 einen neuen Wirt oder eine neue Stelle, an den Kiemen, des alten 



