DEVELOPMENT OF LIMULUS POLYPHEMUS. 177 



into the abdomen. The anterioi- half of the dorsal vessel, with two pairs of arteries and 

 two pairs of valvular openings, is situated in this cephalothorax. Lastly, the genital open- 

 ings in both sexes are situated on the first pair of abdominal lamellate appendages, the 

 testes and ovaries lying wholly in the cephalothorax ; the ovaries, when distended with 

 eggs, filling up the front of the cephalothoracic shield. 



The posterior region of the body, which is evidently the abdomen, consists of nine seg- 

 ments, the long spine-like telson forming the ninth, as plainly seen in the embryo. As an 

 important argument that this portion is a true abdomen, we have seen that the increase in 

 the number of body-segments occurs in this region ; the new segments, and the new ap- 

 pendages joined to them, are interpolated between the third segment (that bearing the third 

 pair of lamellate feet) and the telson. We have before called attention to the fact that in 

 the worms, the Myriapoda (lulus), and in the hexapodous insects, the increase in the num- 

 ber of segments of the body usually takes place between the penultimate and last seg- 

 ments, and this is probably the rule in nearly all the Articulates. (The Trilobites, according 

 to Barrande's researches, apparently form an exception, the thoracic segments being added 

 between the head and the pygidium, the latter apparently consisting of several segments ; 

 but this mode of increase may be due to the peculiar metamorphosis of these Crustacea, 

 the larva being born with the abdomen formed much as in adult life, and the thoracic seg- 

 ments being added at subsequent moults. It should also be borne in mind that the 

 zoeae of the Decapoda also add five thoracic segments at the time of the first moult.) The 

 abdominal cavity of. Limulus is comparatively small, this region being very thin, and, 

 except the termination of the doi'sal vessel and intestine, being mainly filled with the 

 muscles attached to the lamellate feet. The feet are truly branchiopodous, combining the 

 function of respiration with that of swimming, as in the Phyllopoda. 



There are, then, in Limulus no thoracic feet, comparal:)le with those of the Decapoda and 

 the Tetradecapoda, and the thoracic region (as much of it as exists) is merged with the 

 head, in fact never becoming differentiated from the head proper. Thus we have in Limu- 

 lus a crustacean with the body divided into two regions; a cephalothorax (the thorax being 

 potential, when viewed externally, with no appendages or segments to indicate its existence) 

 and a nine-jointed abdomen. 



This disposition of the body segments is paralleled by the structure of the zoea or larva 

 of the Decapoda. 



Turning now to the opinions of different authors as to the affinity of the Eurypteridae to 

 Limulus, and also to the Trilobites, given in Hall's Palaeontology of New York, ra, and 

 Woodward's bibliography in his admirable monograph of the Merostomata, we find that 

 W. Martin, in 1809, " gave a figure and short description of a Limulus crustacean from the 

 coal measures, which he included with the Trilobita." Li 1825, Dr. J. E. DeKay described 

 and figured the first American species known [Eiirypterxis remipes), and referred it to the 

 class Crustacea, and to the order Branchiopoda, so that to him belongs the credit of first cor- 

 rectly estimating the true aflSnities of this animal. In 1844, Professor Agassiz remarked 

 of Pterygotus : " I am rather inclined to believe that this singular animal will become the 

 type of a fiimily intermediate between the Trilobites and the Entomostracans, in which, 

 perhaps, the Eurypteri and the Eidothece will some day be included." In 1846, Burmeister 

 " makes the Eitrypteridoi the first family of the tribe PalceadcB.'" In 1851, Dr. Ferdinand 



MEMomS BOST. SOC. NAT. HIST. VOL. II. 45 



