PYCNOGONOIDEA. 



1387 



posterior to the trunk in their normal relations. If the trunk, like 

 that of Caligus, corresponds to the mandibles and lips alone, then 

 these legs may be analogues of the maxillae, organs which in Caligus 

 are a little remote from the trunk. But if the maxillge are obsolete, or 

 the trunk includes them in its constitution, as is possible and even 

 probable, considering that the maxillae in the Caligoidea are obsoles- 

 cent and never elongated into proper legs, it will then follow, that the 

 legs referred to correspond to the slender didactyle legs (first pair) of 

 Caligus, or the tubular legs of Argulus, and to the maxillipeds (or 

 second pair of maxillae) in the Cyclopoidea and Cyproidea. 



The four pairs of legs which follow, will, on the first supposition, 

 correspond to the first four pairs in Caligus, two subprehensile pairs 

 and two natatory, making the whole number of normal appendages 

 twelve: on the second, to the second or prehensile pair, and three 

 pairs of natatories, making the total fourteen. In Cythere, among the 

 Cyproidea, we find ten appendages behind the antennae, the six pos- 

 terior of which are proper feet, and not natatory appendages ; and this 

 group may perhaps illustrate this point in the structure of the Pycno- 

 gonoicls, rather than the Caligi. In fact, the Entomostraca take this 

 pediform character for the feet and lose the natatory pairs, only 

 among the Cyproidea and Limuli ; and hence we may properly trace 

 the transitions to the terrestrial Articulata, through these few-footed 

 species. 



On this ground, we make out the most probable view of the homo- 

 logies of the parts in the Pycnogonoids to be as follows : — 



APPENDAGES. NORMAL HOMOLOGUES. 



1. First and second pairs. Second antennae, two-branched. 



2. Moveable trunk. Mandibular and first maxillary segment. 



3. Inferior or ovigerous legs. Second maxillary (first pair of feet in Caligus). 



4. 5, 6 ; 7. Pairs of feet. - Next four segments following. 



If we should suppose still another pair of maxillae embraced nor- 

 mally in the moveable trunk, then the four pairs of legs would be 

 homologous with the four pairs of natatories in Caligus and Cyclops, 

 and the inferior legs, with the first or prehensile pair in these genera. 

 But of this we have no evidence. 



The legs in the Pycnogonoids consist each of nine joints, the ante- 

 penult of which is very short, and the last a claw. From the cha- 



