273 M. T. Thorell on the Systematic Position 0/ Me Argulidae. 



ceive that the former constitute a much compacter whole than the 

 latter, and that certain conditions of organization which in the 

 one type (that of the Copepoda) show a considerable degree of 

 constancy are in the other exceedingly variable. Thus the 

 number of body- segments in the Copepods varies within com- 

 paratively narrower limits than in the Branchiopods ; and the 

 same thing is the case, and in a higher degree, with the number of 

 the legs. Since the trunk in Copepods always bears a definite 

 number, at the most (and most frequently) five pairs, of extremi- 

 ties, it would be an extremely remarkable variation if a form 

 appertaining thereto were detected with more than five pairs of 

 legs; whereas it is not at all surprising to meet among the 

 Branchiopods, which certainly are generally distinguished by 

 numerous extremities, with forms possessing only a few pairs of 

 legs (as, for instance, four in the Argulida^) , while their number 

 varies in the Phyllopods from ten to sixty, and in the Cladocera 

 goes down to six, five, or only four pairs. 



The oral organs also are more constant in number in the 

 Copepods than in the Branchiopods, where the maxillary feet 

 are mostly rudimentary or entirely wanting. Even in cases 

 where both pairs are found, as in Ajms, the posterior are always 

 rudimentary — a condition which is seldom observed in the Co- 

 pepods. Amongst these it is usually only in connexion with 

 parasitic habits that the number of the oral organs is reduced ; 

 but it is here worthy of remark that the parasitic nature in 

 gena'al occasions a stronger development of the maxillary feet, and 

 always in those groups at least which possess suctorial tubes and 

 distinctly segmented bodies (i.e. Ascomyzontidje, Caligidse). This 

 would lead us to suppose that if parasitic Branchiopoda be found 

 analogous to the Copepod families just named, the maxillary 

 feet of these, as constituting the seizing-organs already men- 

 tioned, will also have received a stronger development than in 

 the free forms. Notwithstanding, therefore, that the Argu- 

 lidse possess, as is well known, in correlation with their para- 

 sitic habits, particularly strong maxillary feet, this character 

 would not forbid their approximation to the Branchiopoda on 

 the ground of their further agreement with that group. 



In the Copepoda both mandibles and maxillae are usually 

 furnished with a palp, whereas this is never the case in the 

 Branchiopod order, excepting in the genus Nebalia, which 

 difi'ers in so many respects from them, and makes an approach 

 to the Decapoda {Mysis, Cuma). In Copepods the palpi 

 are certainly sometimes wanting, especially in the parasitic 

 forms; but in the higher Siphonostoma (Ascomyzontidse and 

 Caligidse) there is generally at least one pair, while these organs 

 are altogether wanting in the Argulidse. 



