NO. 1788. NORTH AMERICAN ERGASILID^—WTLSON. 281 



bestows upon the corresponding organ in the Caligidse. But A. 

 Scott (1901, pp. 10 and 25) has shown by a careful study of the inner- 

 vation that these organs in Lepeophtheirus are probably the first 

 maxillae, or some portion of them, migrated from their normal posi- 

 tion beside the mouth and transformed into prehensile organs. 

 « He called them the first maxillae, and the other pair close to the 

 mouth the second maxillae. The present author, confirming Scott's 

 observations by an examination of other species of Lepeophtheirus 

 (edwardsi and salmonis) and also certain species of Caligus (rapax 

 and honito), adopted the same nomenclature in dealing with the 

 Caligida? (1905, p. 499). 



But in both instances these names were given upon the assump- 

 tion, put forward by Claus and others, that the two posterior pairs 

 of mouth-parts were the exopod and endopod of one and the same 

 appendage. Hansen, however, discovered in the larvae of certain 

 marine copepoda (Eucalanus, Pontella, etc.), whose body is more 

 elongated than usual, that the two appendages are entirely distinct. 

 Furthermore the posterior pair arise behind the suture line which 

 separates the head from the first thorax segment. Hence they be- 

 long to the latter segment and are true maxillipeds, while the anterior 

 pair become maxillae. These observations have been confirmed by 

 Giesbrecht and by Claus himself upon the same or similar long bodied 

 larvae. They are also confirmed in the present paper upon the larvae 

 of Ergasilus centrarchidarum (p. 323). But this definite proof that the 

 posterior mouth-parts are thoracic, while the pair just in front of 

 them are cephalic, makes one of two things necessary. Either there 

 are three pairs of maxillae in some copepod genera, or the first two 

 pairs are different portions of the same pair. The latter seems much 

 the more probable for several reasons: 1. When completely devel- 

 oped (Calanidae, Pontellidae, etc.) the first maxillae are made up of a 

 distinct endopodite, exopodite, and epipodite, while the protopodite 

 is produced internally into a large masticatory lobe. 2. In all those 

 genera possessing these lateral hooks, the first maxillae consist of but 

 a single one of these parts, or at the most two of them, and are very 

 rudimentary. The lateral hooks might well be one of the other parts, 

 say the exopodite, migrated outward a little toward the lateral mar- 

 gin of the carapace, while the endopod has remained in close prox- 

 imity to the mouth. In the Caligidee the two parts are opposite 

 each other, while here in the Ergasilidae the outer one remains where 

 it first appears in the metanauplius stage and the inner one migrates 

 backward with the other mouth-parts. 3. The two nerves which 

 supply these appendages are distinct to their very origin in Lepeoph- 

 theirus pedoralis according to Scott. In Lepeophtheirus edwardsi 

 they are united for a short distance from their origin, while in some 

 Caligus species they are distinctly branches from a common trunk. 



