DEVELOPMENT OF LIMULUS POLYPHEMUS. I75 



consider that the eyes of Arthropoda ^ generally are not developed upon a separate seg- 

 ment. I have also endeavored to show ^ that in the Insects the compound eyes are devel- 

 oped in the first or antennary segment. 



Examining the embryology of other Branchiopoda, we find that the Cladocera (Daph- 

 nia, according to Dohrn's researches,^ and Leptodora),* and the Phyllopoda (Limnadia)/ 

 have the eyes developed on the same segment, as also Apus ; this may be plainly seen by 

 reference to Zaddach's figures.*' In Artemia salina' they arise as in Apus, on the first anten- 

 nal ring, and afterwards become pedunculated. In Nebalia the figures of Mecznikow show 

 that the eyes are developed on the cephalic plates, as in the Libellulidaj, which form the 

 tergite of the first antennary ring. In the Decapoda, Dohrn (1. c.) has shown that their 

 position is the same, as shown in the embryo of Cuma Goodsiri. 



As seen also in the zoese of the higher Decapoda, these organs arise on the first antennary 

 ring. Dohrn's figures of Praniza maxillaris show the same position of the eyes, and this 

 seems to hold good for all the Tetradecapoda, a view which Rathke's ^ figures of Bopyrus 

 and Ligia confirm. From this evidence we seem justified in considering that the first seg- 

 ment of the head in Arthropoda is antennal. We must, then (assuming, unless future discover- 

 ies reveal an additional pair, that there are but five segments in the head of the Emypterida, 

 each bearing a pair of appendages^), regard the region bearing these members and two pairs 

 of eyes, simple and compound, as a cephalothorax comparable with that of Limulus, the 

 thoracic segments being potential; but with one segment less, that bearing the first pair 

 of maxillipedes. This difi'erence in the number of segments composing this region we 

 would not consider as of much consequence, in a group (Branchiopoda) in which the num- 

 ber of body segments is so inconstant. Mr. Woodward's '"'eighth cephalic segment," 

 bearing the "operculum," or "thoracic plate," and which he himself regards as the homo- 

 logue of the first pair of lamellate feet in Limulus, is, in our view, beyond question the 

 first segment of the abdomen of the Eurypterida3. This is succeeded by twelve segments, 

 counting the telson (as what we now know of the embryology of Limulus gives us a right 

 to do) as one. Thus, from the facts which Mr. Woodward aifords us, we make eighteen in- 

 stead of twenty-one segments, four of his segments being hypothetical.^" Turning now to 

 Professor Hall's restoration of Eurypterus (Palaeontology of New York, vol. lu, 1859), and 

 counting the number of segments in the cephalothorax and abdomen (which I was careful 

 not to do before writing the preceding sentence, in order to test the correctness of my own 

 observations), I find five segments in the cephalothorax, bearing the same number of ap- 

 pendages, and thirteen well defined segments in the abdomen, making eighteen in all, the 



'The term Arthropoda is used simply for convenience, as 'De Apodis cancriformis, etc. 1831. 



I am disposed to regard the division of Articulata into Ar- ' Histoire d'Artemia salina, etc. Annales dcs Sc. Nat. 



thropoda and Worms (Vermes) as a somewhat artificial one. Ser. 2"* , Vol. iii, pp. 225, 840. 



2 Embryological studies on Diplax, Perithemis, and the « Zur Morphologic. Reisebemerkungen aus Taurien. 1837. 



Thysanurous genus Isotoma. Memoirs Peabody Academy ' Huxley's theory, that the antennules, or first antennae of 



of Science, Salem, 1871, p. 21. Pterj-gotus are aborted, appears to me from the reasons sug- 



' Untersuehungen ueber B.au und Entwickelung der Arthro- gested above to be scarcely tenable, having no apparent 



poden. Leipzic, 1870. foundation in the structure of the Branchiopods. 



* See Bidrag til Cladocerernes Forplantnings-historie. '" I can make but two segments out of Mr. Woodward's 



Ved P. E. Miiller. Schiiidte's Naturh. Tidsskrift. 18G8, vii, vill, ix, (see his plate Vlil,) i. e., the first and second ab- 



p. 295. dominal plates, being his "operculum" and the gill-bearing 



' Observations sur la Generation et le Developpement de segment almost concealed by it. 

 la Limnadia Ilermanni. Par M. Lereboullet, Annales des 

 Sc. Nat., Ser. 5°", vol. v, p. 283. 1866. 



