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



blish another view respecting them, tending principally, as it 

 would seem, to discover a nearer affinity between the Argulidae 

 and the Caligidse, in which latter group, as in the parasitic Crus- 

 taceans in general, it is not the first but the second pair of an- 

 tennae which take the form of fixing-organs. According to 

 Kroyer, that which I have called the first pair of antennae con- 

 sists of both pairs of antennae coalesced ; the appendage would 

 thus be the first, the claw itself the second pair of antennae. In 

 accordance with this view, the real second pair of antennae is re- 

 garded as the first pair of footjaws. After what has been already 

 said relative to the history of the development of these organs, a 

 detailed refutation of Kroyer*s hypothesis would be superfluous. 

 With regard to the other appendages of the head, a comparison 

 between them in the Argulidae and the groups allied thereto is 

 rendered difficult by the circumstance that the Argulus-larva. at 

 its exit from the egg is much more fully developed, and already 

 furnished with the parts belonging to the mouth, and also with 

 the two pairs of limbs situated behind these, so that the origins 

 of these parts cannot be followed genetically. One may, how- 

 ever, conclude from analogy that in the Argulus, as in the Phyl- 

 lopods and Copepods, the mandibles are formed out of the base 

 of the second pair of feet in the larva, and the maxillae behind 

 them as an independent pair of limbs. How far the two following 

 pairs of limbs in the Argulidae are originally, asClaus* showed 

 to be the case in the free Gnathostoma and in at least a part of 

 the parasitic Copepoda, branches of one and the same pair of 

 limbs is certainly not clearly ascertained : nevertheless their 

 form and position (especially in the larvae) exhibit too evident 

 marks of correspondence with the footjaws in the higher families 

 of Siphonostoma to admit of their being regarded in their origin 

 and morphological significance as bearing no relation to these. 

 The first pair seems to serve exclusively as fixing-organs : they 

 take, as is known, in the adult state of the difi'erent species of the 

 genus Argulus the form of sucking-cups, while in the larvae 

 and in the genus Gyropeltis they are armed at the extremity with 

 a hook. The functions of the second pair are not equally easy 

 to determine ; but if they serve to maintain the animal when 

 fastened on its prey, they probably also, and perhaps principally, 

 serve as organs of locomotion, and may therefore be called 

 " creeping-feet ^' (pedes gressorii), as they have, indeed, been 

 named by Kroyer. Vogtf maintains that the animal avails 



* Die frei lebenden Copepoden, p. 28 (1.863). "Ueber den Bau und die 

 Entwicklung von Achtheres percarum," Zeitschrift fiir wiss. Zool., Bd. ix. 

 (1861) p. 293. 



t "Beitrage zur Naturgeschicbte der Schweizeriscben Crustaceen," Neue 

 Denkschriftcn der allg. Schweizeriscben Gesellschaft fiir die gesammten 

 Naturwissenschaften, Bd. vii. (1845), p. 12. 



