MORPHOLOGY OF BRAIN AND SENSE ORGANS OF LIMULXJS. 49 



till long after the cephalic lobes, as such^ have disappeared 

 (i. e. beginning of the pupal stage), but it is then impossible 

 to determine exactly their relation to the cephalic lobes. In 

 the Scorpion there is nothing comparable to the convex eye. In 

 Limulus they certainly are not derived from the cephalic lobes, 

 although as in insects the optic ganglion does have this deriva- 

 tion. My recent observations have shown that in Limulus the 

 convex eye arises farther forward than I at first supposed. In its 

 earliest stages it lies about opposite the cheliceral segment to 

 which it in all probability belongs ; its subsequent union with 

 the optic ganglion of the cephalic lobes is therefore a secondary 

 affair. The same thing apparently takes place in Acilius, for 

 the convex eyes when they appear at the beginning of the 

 pupal stage, instead of being provided with a special 

 ganglion of their own, become united with the 

 ganglia of the degenerative ocelli. 



These facts suggest a very interesting comparison with 

 Myriapods. There seems to be present in nearly all Myriapods 

 a remarkable nerve arising from the optic ganglion and supply- 

 ing a peculiar sense organ situated at the base of the antennae. 

 St. Remy calls them the nerve and sense organ of Tomosvary 

 (fig. 62). Now I strongly suspect that this sense organ is the 

 rudiment of the convex eye of the higher Arthropods, for it 

 agrees in two important particulars with the convex eye of 

 Limulus, and presumably with that of all other Arthropods, 

 namely, in its situation at the base of the antennae, and in the 

 attachment of its nerve to the ganglion of the larval ocelli. 



In Acilius, the compound eyes appear at the beginning of 

 the pupal stage as a sickle-shaped band on the dorsal and 

 median margin of the ocelli. They finally breakaway from the 

 surface, and their degenerated remains become attached to the 

 under side of the optic ganglia (fig. 60). The latter persist, 

 and form the ganglion of the compound eyes. The conversion 

 of the larval optic ganglion into that of the imago is brought 

 about in the pupal stage by rapid growth along three very 

 clearly marked regions (fig. 60, o. g. 1 — 3). In all three 

 bands, or centres, which probably represent the three segments 



VOL. 35, PART 1. — NEW SER. D 



