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A. S. PACKARD, JR., OK THE 



sists of a single piece. ^ The mandibles, as in the Insecta,have no palpi. The maxillipedes, 

 of which the third pair is often still wanting, are not yet brought into the service of the 

 mouth, but appear in the form of biramose natatory feet. Branchiae are wanting, or when 



their first rudiments may be detected as small verruci- 

 form prominences, these are dense cell-masses, through 

 which the blood does not yet flow, and which, therefore, 

 liave nothing to do with respiration. Fig. 29, (from Miil- 

 lor) is an example of a zoea, being the "yovmg zoea" 

 of Penaeus. As in the zoea, the body of the larval 

 Limulus consists of a head and abdomen, the thoracic 

 segments not being at this or at any subsequent stage 

 developed ; for in the zoea before the first moult, the 

 thorax is potential, the rudiments of five thoracic seg- 

 ments being seen just behind the postei'ior pair of decid- 

 uous maxillipedes. The head is comparable to the 

 ceijhalic shield of Apus ; like it, bearing a pair of simple 

 and compound eyes, but greatly differing from it in being 

 united with the body along the lower edge, instead of 

 being free, as in the Phyllopoda. In the zoea stage of 

 Decapoda, however, the eyes are not at first pedunculated, 

 but sunken in the carapace. The six appendages of Lim- 

 ulus are cephalic, the first two pairs being probably the 

 homologues of the first and second pair of antenmc" of 

 the zoea of Decapoda, and tlie Phyllopoda (Apodida3 and Branchipodidse), Avhile the third 

 pair probably represent the mandibles, the fourth pair the first maxillae, and the last two 

 pairs the maxillipedes (perhaps the fifth pan- represent the second maxillae of Apus, and 

 consequently the sixth pair alone represent the maxillipedes).'' It should be noticed 

 that these appendages are all situated in front of the metastoma, which in Limulus has 

 been homologized with the languette or '-labium " of the Decapoda, by Milne Edwards.* 



^ I have found an exception to this in the zoea of Alpheus 

 minus Say, observed in January at Key West, Florida, in 

 which, before it is hatched, there are five pairs of short bira- 

 mose abdominal feet. The absence of abdominal appendages 

 is not, tlicrefore, an essential character. 



2 At first I was disposed to consider the first pair of ap- 

 pendiiges of Limulus as mandibles, regarding the antennfe as 

 wanting. (See ''American Naturalist," iv., Feb., 1871, p. 

 754.) Fabricius (1783) called them mandililes, considering 

 Limulus as wanting antenniE. Savigny (1816) regarded Lim- 

 ulus as %vanting, not only the two pairs of antenna, but also 

 the four succeeding pairs of limbs. He calls the first pair of 

 actual appendages '•mandibules succedanees," and compares 

 them with the third pair of maxillipedes of Apus. Van der 

 Hoeven (1838) calls them feet, while Latreille at first (Hist. 

 Nat. des Crust. Ins., 18) followed Fabricius in regarding them 

 as mandibles, and afterwards, as Van der Hoeven states, he 

 doubtfully considered them as antenna>, in the first edition of 

 the "Eegne Animal." But in his "Families Naturelles" (1825) 

 and the second edition of the "Regno Animal," (1829) he 

 considers them as representing the antennae of other Crus- 



tacea, though usually in referring to them he calls them feel 

 (pattes). 



^ I find that Mr. Woodward has applied the same terms to 

 the appendages (see his work on the British Fossil Merosto- 

 mata). The fact that all the appendages of the nauplius of 

 the Cirripedia are situated in front of this prolabium, and that 

 its position is identical with that of Limulus, is another proof 

 that the ambulatory appendages of Limulus should be all 

 considered as cephalic. 



•"Latreille (Regne Animal, 2d Edition, 1829) remarks of 

 this double organ, " Les deux pieces situees dans I'entre-deux 

 de CCS pattes, que M. Savigny considere comme une languette 

 ne me paraissent ctre que deux lobes inaxillaires de ces or- 

 ganes, mais detaches an libres." p. 186. But we have shown 

 that this metastoma arises as two tubercles long after the 

 last pair of cephalothoracic feet are developed, being en- 

 tirely disconnected from, and forming no part of them. Tliis 

 ortran seems to be homologous with the immense bifid organ 

 of the nau]>lii of certain Cirripedia (especially Tctraclita, 

 as figured by Fritz MuUer), which nearly equals the abdomi- 

 nal spine in size. 



