58 WILLIAM PATTEN. 



canal. I maintained, in my paper on "Acilius/' that the 

 segments of the cephalic lobes of Insects were originally post- 

 oral, and comparable with those in the trunk. While T still 

 recognise the great similarity of trunk and cephalic segments, 

 I do not now believe that they are morphologically identical. 

 The constant presence of the larval ocelli, and the absence of 

 typical appendages, besides other considerations, indicate a 

 sharp demarcation between the segments of the cephalic lobes 

 and those of the body. 



III. Development of the Brain of Limulus. 



A. The cephalic lobes of Limulus, before they divide 

 into segments, resemble more closely in shape and general 

 appearance those of Insects and Scorpions than those of Crusta- 

 ceans. We find there the same three pairs of invaginations 

 seen in Acilius and Scorpions, but so strangely modified as to 

 be at first sight hardly recognisable. 



The first change that takes place is the formation of a great 

 furrow along the whole of their anterior margin. As the 

 furrow deepens, the lobes become a little shorter, and assume 

 the shape shown in fig. 61 ; at the same time they come to be 

 diff'erentiated into four distinct swellings. The lateral ones 

 constitute the optic ganglia [op. g.), or, as I shall sometimes 

 call them, the thalamencephalon, owing to their double 

 relation to the nerves of the lateral eyes and olfactory organ, 

 and to the way they finally become incorporated into the adult 

 brain. In front of the optic ganglia the furrow becomes 

 deeper and broader, and constitutes the invaginations of the 

 optic ganglia. Just in front of the latter, and united with 

 them by thick nerves, is a pair of oval thickenings, the primitive 

 olfactory organs [p. ol. o.). The compound eyes, with which this 

 optic ganglion is subsequently united, have not yet appeared. 

 They are first seen near the outer end, and a little back of, the 

 ganglion, as though they belonged to the cheliceral segment. 



The optic ganglia of Limulus evidently correspond to the 

 optic ganglia of the third segment of Scorpions and Insects, 



