NERVOUS SYSTEM AND SENSORY ORGANS. 



351 



a posterior thoracic segment (according to Packard and Patten 

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

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

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

 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 {pm) arises in the form of a 



^rm% 





H. 





y^^li^- 



Fig. 1C3. — Transverse section through the Trilobite stage of Limulns (after Watase). hg, 

 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 be considered as very 

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

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

 exceptional among the Arthropoda. Although the statements of Patten and 

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



