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"Four stages in the development of the trilobite Agnostus nudus. A, Youngest stage with no mesosomatic somites; B and C, stages with two mesosomatic somites between the prosomatic and telsonic carapaces; D, adult condition, still with only two free mesosomatic somites." — The Encyclopedia Britannica, 1910

Agnostus Nudus

"Four stages in the development of the trilobite Agnostus nudus. A, Youngest stage with no mesosomatic…

"The remaining three pairs of mesosomatic appendages of Scorpio and Limulus. l130 indicates that there are 130 lamellae in the scorpion's lung-book, whilst l150 indicates that 150 similar lamellae are counted in the gill of Limulus." — The Encyclopedia Britannica, 1910

Limulus

"The remaining three pairs of mesosomatic appendages of Scorpio and Limulus. l130 indicates that there…

"Ventral view of Limulus polyphemus. 1 to 6, The six prosomatic pairs of appendages. abd, the solid opisthosomatic carapace. tels, the post-anal spine (not the telson as the lettering would seem to imply, but only its post-anal portion). operc, the fused first pair of mesosomatic appendages forming the genital operculum." — The Encyclopedia Britannica, 1910

Limulus Polyphemus

"Ventral view of Limulus polyphemus. 1 to 6, The six prosomatic pairs of appendages. abd, the solid…

"The first three pairs of mesosomatic appendages of Scorpio and Limulus compared. VII. The genital operculum; VIII. The pectens of Scorpio and the first branchial plate of Limulus; IX. The first pair of lung-books of Scorpio and the second branchial plate of Limulus. gp, Genital pore; epst, Epistigmatic sclerite; stg, Stigma or orifice of the hollow tendons of the branchial plates of Limulus." — The Encyclopedia Britannica, 1910

Mesosomatic

"The first three pairs of mesosomatic appendages of Scorpio and Limulus compared. VII. The genital operculum;…

"Five stages in the development of the trilobite Sao hirsuta. A, Youngest stage. B, Older stage with distinct pygidial carapace. C, Stage with two free mesosomatic somites between the prosomatic and telsonic carapaces. D, Stage with seven free intermediate somites. E, Stage with twelve free somites; the telsonic carapace has not increased in size. a, Lateral eye. g, So-called facial "suture" (not really a suture). p, Telsonic carapace." — The Encyclopedia Britannica, 1910

Sao Hirsuto

"Five stages in the development of the trilobite Sao hirsuta. A, Youngest stage. B, Older stage with…