M. T. Thoi-ell on the Systematic Position of the Argulidse. 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 Argulidse 

 then known {A. foliaceiis) held a place in the ' Syst. Nat/ ed.lO, 

 between M. poIyphemus 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 Midler*, 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 Argulidse 

 has been shared by most of the more modern authors, as Bur- 

 meister, Milne-Edwards, Baird, Dana, Heller, Coruaha, Claus, 

 Kroyer, &c. As Latreille's Siphonostoma and Milne-Edwards's 

 Lerneides have been united with that author's Copepodes in a 

 single order by Zenkerf, 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 Argulidse have been sought. But this 

 view of the affinities of the Argulidse 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 Argulidse and the true Siphonostoma, and express 

 their opinion that Argulus should perhaps hereafter become the 

 type of a new order, standing between the Siphonostoma and 

 the Pcecilopoda or Xiphura. Their attempt to show a nearer 

 relationship between the Argulidse and these last must, however, 

 be regarded as abortive, since it is pretty generally recognized 

 that such conditions of organization as may ally the Argulidse 

 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, qua; in aquis Danise et Norvegise 

 reperit, &c. O. F. Muller (1785), p. 121. 



t Untersuchuugen iiber die Organisation und Verwandtschaft der Co- 

 pcpoden, p. 64. 



