M. T. Thorell on the Systematic Position of the Argulide. 269 
to the genus Monoculus (which corresponds with almost all the 
lower Crustacean orders or “ Entomostraca” with the exception 
of the Cirripeds), the sole representative of the family Argulidz 
then known (4. foliaceus) held a place in the ‘ Syst. Nat.’ ed.10, 
between M. polyphemus and M. apus, consequently between a 
Limulus and an Apus. In the ‘ Fauna Suecica,’ ed. 2, a Caligus 
(M. piscinus, L.) was located between Argulus and Apus. In 
the ‘ Entomostraca’ of Miller*, who first erected the genus Ar- 
gulus, we find it between Polyphemus and Limulus, immediately 
after which comes the genus Caligus. 
From this time, however, it gradually became general to place 
Caligus and Argulus near to each other, especially since Latreille 
laid the foundation of the present prevalent division of the class 
of Crustacea, and constituted the order Siphonostoma for those 
forms which are provided with sucking-tubes, placing therein 
the genera Argulus and Caligus in close proximity to each 
other. This view of the systematic position of the Argulide 
has been shared by most of the more modern authors, as Bur- 
meister, Milne-Edwards, Baird, Dana, Heller, Cornalia, Claus, 
Kroyer, &e. As Latreille’s Siphonostoma and Milne-Edwards’s 
Lernéides have been united with that author’s Copépodes in a 
single order by Zenker+, under the name Entomostraca, called 
Copepoda by later authors, it is consequently among the para- 
sitic suctorial Copepoda (Copepoda siphonostoma) that the 
nearest affinities of the Argulide have been sought. But this 
view of the affinities of the Argulide has been by no means 
generally accepted: from more than one quarter have objections 
to it been put forward, which render necessary a further exami- 
nation of the reasons alleged in favour of it. Dana and Herrick 
have already pointed out the great differences between the oral 
organs of the Argulide and the true Siphonostoma, and express 
their opinion that Argudus should perhaps hereafter become the 
type of a new order, standing between the Siphonostoma and 
the Peecilopoda or Xiphura. Their attempt to show a nearer 
relationship between the Argulidz and these last must, however, 
be regarded as abortive, since it is pretty generally recognized 
that such conditions of organization as may ally the Argulidz 
to Limulus are presented in a still more marked degree by the 
Phyllopoda, especially the genus Apus. Limulus shows, more- 
over, so many remarkable peculiarities in its structure, that it 
must necessarily be regarded as the type of a separate order, in 
spite of its being more nearly related to the Branchiopoda and 
* Entomostraca seu Insecta testacea, que in aquis Dani et Norvegie 
reperit, &e. O. F. Miiller (1785), p. 121. 
+ Untersuchungen iiber die Organisation und Verwandtschaft der Co- 
pepoden, p. 54, 
