252 Description of a Species of Caligus, 



These seven pairs appear to correspond to those of the first tho- 

 racic ganghons in the macrural Crustacea. The remaining nerves 

 pertain to the thoracic legs, and the abdominal portions of the 

 body.* 



The outer pair (1) belong to the anterior natatories. They 

 continue parallel with the spinal cord till they reach the furcate 

 process on the venter ; they then curve outward, exterior to the 

 ventral muscles, give off three branches in succession from the 

 outer side, to the muscles of the first natatory. Before entering 

 the basal joints of these legs, they divide into three portions, 

 which enter together ; the inner branch is quite slender, and pass- 

 es to the posterior movable seta, and the jointed appendage ; the 

 middle is distributed to the musclec of the basal joint ; the outer 

 branch gives a slender nerve to the apex of the basal joint, and 

 then passes to the two following joints, dividing as it enters them. 

 We refer for minuter details to figure 18, Plate V, 



This pair of nerves give off a slender branch near their origin, 

 (r, fig. 20,) which passes to the attachments of the stomach. 



The next pair of nerves (m) are distributed to the second pair 

 of natatories. They diverge from the spinal cord — to which they 

 are adjacent — below the furcate process, and soon give off a branch 



* The apparent correspondence of these nerves, with those of the first thoracic 

 ganglion in the higher Crustacea, together with the great similarity in the last two 

 pairs, to those which are distributed to the posterior pair of maxillipeds in the 

 decapodous species, (see Edwards's Hist. Nat. des Crustaces, T. I. p. 137) have 

 induced us to adopt the designations heretofore employed in speaking of the organs 

 of this segment of the body, in which we consider the three posterior pairs of ce- 

 phalic legs as the anologues of the three pairs of maxillipeds in the typical species. 

 The only objection which can be urged to this view, arises from there then being 

 h\xt four pairs of thoracic legs. This objection will however be removed if we may 

 consider the furcate bone on the venter, just anterior to the first pair of natatories, 

 as the rudiment of the sternum of a fifth pair, which view is favored by its posi- 

 tion, and its resemblance to the sternums of the following pairs. 



If this conclusion is correct, the twenty-one rings, the normal number consti- 

 tuting the body, may be considered as distributed in the following manner : 



The anterior cephalic segment, includes the second and third, as the first — the 

 ophthalmic — is wanting. The posterior cephalic segment, contains the following 

 six, corresponding to the mandibles, ('fourth,) and a pair of maxilla, (fifth,) in the 

 buccal mass ; and the four pair of feet attached to this segment. The anterior 

 thoracic segment will include the tenth, eleventh, twelfth and thirteenth rings, to 

 which the furcate process, the two pairs of natatory legs, and the apron, pertain. 

 The posterior thoracic segment, is the fourteenth ring. The remaining seven rings, 

 the abdominal, constitute the terminal portions of the body; some of these last 

 may however be wanting. 



