Fig. 148.—Section through the three Sense-Organs of Flabellum of Limulus. "bl.", blood-spaces; "n.", nerve; "gl.", layer of ganglion-cells; "p.", pigment layer; "ch.", 1, 2, 3, the three layers of chitin; "ch.t.", chitinous tubule in large tube of sense-organ; "cap.", capitellum or swollen extremity of large tube; "can.", very fine porous canals or canaliculi of chitin.