46 WILLIAM PATTEN. 



quently, is formed by the uuion, over the crystalline cones, of 

 two cells derived from the sides of each ommatidium. The 

 same process undoubtedly occurs in Crustacea. 



I studied the subject again in Acilius, and fortunately this 

 insect furnished the most primitive and least modified type of 

 cephalic lobes yet described, and apparently the one from 

 which all others have been derived. I do not by any means 

 believe that the Coleoptera are the most primitive Arthropods. 

 But since we find the Acilius type of fore-brain clearly 

 repeated with more or less modification in other insects, in 

 Spiders, Scorpions, Limulus, and probably with still greater 

 modifications in the Crustaceans (although our knowledge of 

 the last group is too imperfect as yet to speak definitely about 

 them), it must be the nearest in structure to the ancestral type. 



A. The Cephalic Lobes of Insects. — Basing our con- 

 clusions mainly on Acilius, we find that in insects the true 

 fore-brain is derived from the cephalic lobes, which 

 are composed of three segments. In each segment one 

 can readily distinguish a pair of brain-lobes, a pair of optic 

 ganglia, and two ocelli opposite each ganglion (PL 5, fig. 57). 

 Between the ocelli and the ganglia are three pairs of invagina- 

 tions, which decrease in depth and extent from the first to the 

 third. The ocelli, after the closure of the invagination, still 

 remain on the margin of the cephalic lobes in their original 

 upright position. 



The antennary neuromere is usually regarded as a part of 

 the cephalic lobes. I believe this is a mistake, as can be very 

 readily shown by comparison with Scorpions and Limulus. One 

 finds there, as well as in Acilius, no evidence whatever that it 

 forms part of the true cephalic lobes; but it is not possible to 

 show this without the proper material properly prepared. I 

 regard the antennary neuromere as morphologically post-oral 

 — it is unquestionably derived from the ventral cord of the 

 trunk. It moves gradually forward till it occupies a position 

 beside, or in front of, the mouth, constituting what I shall call 

 the mid-brain. 



