NERVOUS SYSTEM AND SENSORY ORGANS. 



351 



a posterior thoracic segment (according to PACKARD and PATTEN 

 the third, according to KINGSLBY the fifth; cf. also Fig. 157, 

 au and do). The optic area proper (Fig. 163, la) is bounded at its 

 dorsal (median) and ventral (lateral) margins by folds (df and vf), 

 which, converging posteriorly till they meet, form the letter V. At 

 the point at Avhich the two folds meet, a short tubular invagination 

 of the ectoderm is formed, extending below the surface, so that the 

 V-shaped rudiment becomes Y-shaped. These folds, which are 

 composed of very large cells (Fig. 164, df and vf), yield new cell- 

 material for the formation of younger ommatidia at the margin of 

 the optic area. Each ommatidium (om) arises in the form of a 



la. 



Fio. 163. Transverse section through the Trilobite stage of Limulvs (after WATASE). bg, 

 ventral chain of ganglia; d, food-yolk (rudiment of enteron); do, dorsal organ; df, dorsal 

 fold, and vf, ventral fold of the rudiment of the lateral eye (la); ent, endosternum ; ex, 

 limb-rudiment. 



simple depression of the ectoderm (hypodermis), over which the 

 cuticle becomes thickened to form a conical lens (c). The manner 

 in which the optic nerve of this lateral eye arises is not quite clear. 

 PATTEN and KINGSLEY trace it to the nerve-strand of the lateral 

 sensory organs (p. 350). 



The position of the lateral eyes of Limulus must he considered as very 

 remarkable. Authors are unanimous in attributing these eyes to a post-oral 

 thoracic segment of the body. They would thus have a position altogether 

 exceptional among the Arthropoda. Although the statements of PATTEN and 

 KINGSLEY just given would to a certain extent explain this, it must appear 



