153 M. T. Thorell on the Morphology of the Argulidse. 



differently regarded, this is certainly the case in a like or even 

 greater degree with the accessory organs, especially the anterior 

 ones or those which appertain to the head. We shall treat of 

 those which belong essentially to the mouth further on, and 

 would begin by directing our attention especially to the four 

 pairs of members which are situated before and behind these. 

 The views taken of these organs, of which we call the two fore- 

 most pairs the first and second pairs of antennce, and the two 

 hindmost the^r*^ and second pairs of footjaws, have, as we have 

 said, been very various, doubtless through erroneous notions of a 

 complete correspondence between the accessory organs of the head 

 in the lower Crustaceans and in the Decapoda, which has rendered 

 the terminology of the former so confused and contradictory*. 

 Of the antennce one pair has usually been considered a pair of 

 footjaws, — the first pair by Heller and Cornalia, the second by 

 M, -Edwards and Kroyer. The view we have taken agrees with 

 that given by Dana and Herrickf ; and its correctness is shown, 

 not only by the form and position of the corresponding parts in 

 the Phyllopods and Copepods, to which the Argulidse are most 

 nearly allied, but also by the history of their development. The 

 newly hatched larva of Argulus has, as Jurine'sJ and also 

 Dana^s and Herrick^s figures attest, a pair of antennge and two 

 pairs of swimming-feet, like the larvae of the Phyllopods and 

 Copepods ; and since the organs which are developed from the 

 antennae and first pair of jaws in the Phyllopod and Copepod larvae 

 ai'e now generally regarded as the first and second pair of antennae, 

 the same rule should be applied to the Argulidae. A glance at 

 the larva of an Argulus shows immediately that the conditions are 

 here just the same as in the case of the Phyllopod and Copepod 

 groups ; the sole difference between the Argulidae and these is 

 that the antennae in the Argulus-\sirYa gradually attain a hooked 

 form, becoming hooked fixing-organs through a stronger deve- 

 lopment of their two first joints, while the other joints are corre- 

 spondingly reduced and at last form only a small appendage to 

 the second joint of the antennae. The first pair of feet in the 

 larva is, as is usual in the allied Crustacea, biramose ; the hinder 

 branch already exhibits the form of the adult animal's second 

 pair of antennae; the foremost branch disappears during the de- 

 velopment of the larva. 



I should not have dwelt so long on the antennae of the Argu- 

 lidae were it not that Kroyer § has very lately sought to esta- 



* Vide Claus, " Zur Morphologic der Copepoden," Wiirzburger Natur- 

 wissenschaftliche Zeitschrift, Bd. i. (1860) p. 26 &c. 



t Descr. oiArg, catostomi, Silliman's Journal, vol. xxxi. (1837) p. 298 &c. 

 X Memoire sur V Argule foliace, Ann. du Mus. t. vii. (1806) pi. 26, fig 4. 

 § Loc. cit, p. 87. 



