272 M.T. Thorell on the Systematic Position of the Argulide. 
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 Argulide), 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. ven in cases 
where both pairs are found, as in Apus, 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 
general occasions a stronger development of the maaillary feet, and 
always in those groups at least which possess suctorial tubes and 
distinctly segmented bodies (i.e. Ascomyzontide, Caligide). 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- 
lidze 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 maxille are usually 
furnished with a palp, whereas this is never the case in the 
Branchiopod order, excepting in the genus Nebalia, which 
differs 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 (Ascomyzontide and 
Caligidz) there is generally at least one pair, while these organs 
are altogether wanting in the Argulide, 
