M.T. Thorell on the Systematic Position of the Argulide. 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 Argulide, I suppose he means the so-called shell-gland 
(skalkérteln) 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 
Cladocera}, in the Cytheride among the Ostracodat, 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 antenne ; and Kroyer has 
a similar suggestion where he says § that in the Caligide the 
claw on the second pair of antennz ‘ 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. Jf, meanwhile, these suggestions of 
Zenker and Kroyer are correct, and 2f the “ shell-” or “ poison- 
* Zenker, “Ueber die Cyclopiden des siissen Wassers,” Archiv fiir 
Naturgeschichte, xx. (1854) p. 98; also Claus, loc. cit. p. 60. 
t+ Leydig, Naturgeschichte der Daphniden, p. 23 &e. 
{ Zenker, “ Monographie der Ostracoden,”’ Archiv fiir Naturgeschichte, 
xx. (1854) pp. 18 & 29. 
§ Loe. cit. p. 105. 
Am. & Mag. N. Hist. Ser. 3. Vol. xviii. 20 
