DEVELOPMENT OP LIMULUS POLYPHEMUS. 179 



Hall's direct homology (Palaeontology, N. Y., iii, pp. 298, 299) of the first pair of ab- 

 dominal feet of Eurypterus with those of Limulus, also his discovery of the ocelli and the 

 true number of joints in the fifth pair of feet of Eurypterus (thus demonstrating more 

 fully than ever before the close affinity of these two groups, at first sight so unlike), and 

 Huxley's comparison of Eurypterus to the zoea of the Decapoda, are perhaps the two most 

 brilliant discoveries we have to record in the history of our knowledge of the Merostomata, 

 since the original discovery of the true affinities of Eurypterus by DeKay in 1825. 



But while Professor Huxley has made such an apt comparison of Pterygotus to a zoea, 

 he seems to have adopted certain taxonomic views directly contrary to Prof. Hall's, and 

 which seem tons quite untenable. Mr. Woodward ^ quotes him as saying in 1859 ("On 

 the Anatomy and Affinities of the Genus Pterygotus, Memoirs of the Geological Survey of 

 Great Britain, Monograph I"), "The Poeciloptera [i. e., Limulidse] are, I believe, the only 

 Crustacea which possess antennary organs like those of Pterygotus, and, like them, have 

 the gnathites converted into locomotive organs, want the appendages to the sixth abdom- 

 inal somite, and present on some parts of the body a remotely similar sculpture. In this 

 order, however, we find but a small labrum, a rudimentary metastoma, a very differently 

 constructed body, and a larger number of appendages, both thoi'acic and abdominal — 

 characters which effectually preclude the association of the extinct Crustacea uuder dis- 

 cussion with this type." He adds in a foot note^ " If the abdominal somites of the carbo- 

 niferous Bellinui'us, etc., were really free, they would present a certain approximation to 

 the Pterj'-gotus. Indeed, the evidence that these carboniferous Crustacea wei^e true Poeci- 

 lopoda is, to my mind, any thing but conclusive." That such an acute mind as Profes- 

 sor Huxley's could separate (if we understand him aright) Bellinurus from Limulus seems 

 strange. The characters he enumerates as distinguishing the Poecilopoda from other 

 Crustacea are, without an exception, of secondary importance. Apus has one pair of 

 limbs serving at the base as chewing organs (gnathites, according to Huxley), and at the 

 extremity as locomotive organs. The size of the labrum, or hypostoma, varies greatly in 

 the Branchiopoda, and especially the Phyllopoda. The body is very differently constructed 

 in different genera of the Phyllopoda, and the number of " appendages, both thoracic and 

 abdominal," varies indefinitely in the Branchiopoda. 



Woodward, in his splendid "Monograph of the fossil Merostomata (1866)," adopts "the 

 advanced ideas of Agassiz, McCoy, Hall, Huxley, Salter and others," and shows the close 

 relationship of the Eurypterida? to the Limulidae, placing them as suborders [Eurypterida 

 Huxley, and Xiphosura Grouovan) of the order Merostomata of Dana. To his work we 

 are mostly indebted for a full summary of our present knowledge of the fossil Merostomata, 

 and I have made frequent use of it in the preparation of this paper. 



The Rev. Dr. Lockwood, "American Naturalist," iv, p. 269, compares certain features in 

 the larval stage of Limulus, with Pterygotus and Eurypterus. 



The larva of Limulus zoiiivform. Fritz Miiller, in his suggestive work "Fiir Dar- 

 win," thus enumerates the most important peculiarities which distinguish the zoea, or larva, 

 from the adult decapodous animal : "The middle body, with its appendages, those five pairs 

 of feet to which these animals owe their name of Decapoda, is either entirely wanting, or 

 scarcely indicated. The abdomen and tail are destitute of appendages, and the latter con- 



^ " On some Points in the Structure of llie Xiphosura, rida>." (From t!ie Quarterly Jourual of" the Geological Soci- 

 having reference to their Relationship with the Eurypte- ety, Feb., 18G7, p. 28.) 



