282 M. T. Thorell on the Systematic Position of the Argulidse. 



gland " in Argulus opens through the " sting," which has not 

 yet been attested from observation, then it is only the spine 

 just mentioned on the second pair of antennae in the Caligidse 

 and Cytheridae (consequently in animals belonging to two dif- 

 ferent orders) which can be compared with the sting in Argulus. 

 As the " poison-gland " itself occurs in many widely separated 

 orders, it is easy to see that its presence in the Argulidse in no 

 way points out their zoological affinity; and should it open 

 through a spine in some parasitic Copepoda, these would be in 

 the same relation to Cyclops and Cyclopsine (which lack such an 

 organ) as that which the parasitic Argulus with sting bears to 

 the free Branchiopoda without sting. 



Kroyer produces two further reasons for his view of the sys- 

 tematic position of the Argulidse, which it now remains for us 

 to remove. The first is " the absence in Argulidse of external 

 egg-sacs — which finds its analogy in the genera Notodelphys, 

 Doropygus, &c." It seems to us, on the contrary, quite obvious 

 that the absence of external egg- sacs goes to prove that the 

 Argulidae are not Copepods, since these, with the exception of 

 the Notodelphyidae and Buprorus, generally have external egg- 

 sacs, as is also the case in particular with the Caligidce and all 

 the other Siphonostoma with which the Argulidse would be ranged 

 were they Copepods. The asserted analogy is, moreover, very 

 feeble ; for in the Argulidse the eggs stand on the ovary itself 

 until they attain freedom, whereas in Notodelphys &c. they 

 pass from the ovaries into a matrix comparable with the 

 so-called uterus in the Branchiopoda, or, still better, with the 

 matrix of the Cladocera. 



Kroyer finds the last attestation of the Copepod nature of the 

 Argulidse in " the simple eyes placed in a triangular form, which 

 recur not only in the free-swimming Copepods [Sapphirina] , 

 but also in the parasites in the larval state." With reference to 

 this, we have only to remark that an unpaired eye, retained after 

 the larval period, without or with two, three, or several crystal- 

 line bodies ("simple eyes"), is quite usual, not only among the 

 Copepoda (where it generally constitutes the sole visual organ), 

 but also among Phyllopoda, Cladocera, and other lower Crusta- 

 ceans, and that consequently the presence of such an eye in 

 Argulus in no way proves its relationship to the Copepoda. 

 Further, Kroyer's representation of the structure of this single 

 eye in Argulus is incorrect; for what he calls three simple eyes 

 are in that animal a three-lobed prolongation of the brain itself, 

 bearing a pigment-spot, in which not a trace of crystalline bodies 

 or " simple eyes " is to be detected, at least in either A. fulia- 

 ceus or A. coregoni. The unpaired eye in many Branchiopoda 

 (as Branchipus and Artemia) also shows itself as such a pigment- 



