M. T. Thorell on the Systematic Position of the Argulidse. 281 



Whether the animals be related or not, the structure of their 

 mouth must necessarily be somewhat similar in appearance. 

 Circumstances which are modified in correspondence with pecu- 

 liar modes of life are not signs sufficient to determine original 

 affinity ; better as such are those taken from characters which 

 are as far as possible independent of the peculiarities of habits 

 and mode of life/' 



The presence, therefore, of a suctorial organ shows merely 

 that Argulus is a parasitic Crustacean, but does not point out 

 to which order it should be referred. With reference to the 

 *^ sting,'' Kroyer says that it "assuredly corresponds to the 

 poison-weapon in many of the lower Crustacea, both free-swim- 

 ming Copepods (as Cyclopsine castor) and parasites, although 

 peculiar both in form and position." As Kroyer here mentions 

 Cyclopsine castor, which has no organ comparable to the sting 

 of the Argulidse, I suppose he means the so-called shell-gland 

 (skalkorteln) which has not only been observed in Argulus and 

 some Copepods*, amongst which is Cyclopsine castor, but which 

 occurs generally in the Branchiopoda, both Phyllopoda and 

 Cladoceraf, in the Cytheridse among the Ostracoda J, and which 

 is considered to be the same organ as that known as the green gland 

 in Decapoda and Amphipoda. Meanwhile the determination of 

 this " shell-gland " as a secretory and specially as a poison-organ 

 is in the highest degree uncertain, and the more so since we do 

 not as yet know for certain whether any channel exists in con- 

 nexion with it. Zenker certainly insists that it opens externally in 

 Cythere through a spine on the lower antennae ; and Kroyer has 

 a similar suggestion where he says § that in the Caligidse the 

 claw on the second pair of antennte " shows on the concave side 

 very frequently (perhaps always) a bristle or fine spine, which 

 seems to be connected with an extensive internal apparatus 

 (gland, channel, and bladder) ." But he continues thus : — 

 *' Whether this is to be referred to the category of the organs 

 lately pointed out in many of the lower Crustacea, and desig- 

 nated poison-weapon, must be left undetermined," and adds that 

 he "has often found a perfectly similar apparatus on the hooked 

 second pair of feet," — an addition which renders the propriety 

 of a comparison with the " poison-organ" of, for instance, Cy- 

 clopsine very doubtful. If, meanwhile, these suggestions of 

 Zenker and Kroyer are correct, and if the " shell-" or " poison- 



* Zenker, "Ueber die Cyclopidea des siissen "Wassers," Archiv fiir 

 Naturgeschichte, xx. (1854) p. 98 ; also Claus, loc. cit. p. 60. 



t Leydig, Naturgeschichte der Daphniden, p. 23 &c. 



X Zenker, " Monographic der Ostracoden," Archiv fiir Natui'geschichte, 

 XX. (1854) pp. 18 & 29. 



§ Loc. cit. p. 105. 



Ann. ^ Mag, N, Hist. Ser. 3. Vol, xyiii. 20 



