328 INSECTA. 



seem to confirm Patten's views. Heider (No. 38) and Geaber (No. 30), 

 however, although convinced of the presence of a primary segmentation of the 

 brain (b l -b 3 ) in Hydrophilus, were unable clearly to recognise it in the optic 

 ganglion and the optic plate. A comparison with the condition in other 

 Arthropoda, especially in the Crustacea (Vol. ii., p. 162 ct seq.), also supports 

 the view that the optic ganglion is a secondary section of the brain belonging 

 exclusively to the most anterior part. This view is in agreement also with the 

 recent statements of VlALLANES (No. 84) with regard to Mantis. According to 

 this author, the primary part of the brain breaks up into three sections, corre- 

 sponding to the protocerebrum {pc), the deutocerebrum (dc), and the tritocerebrum 

 (ic) of the adult. Of these, the protocerebrum is connected with the optic 

 ganglion {go) and also yields the nerves to the ocelli {no), as well as the dorsal 

 integumentary nerves ; the deutocerebrum yields the antennal nerves {na' and 

 na"), while the tritocerebrum gives off the labro-frontal nerves {If) which are 

 connected with the frontal ganglion. In the rudiment of the optic ganglion, 

 Viallanes could only recognise a division into an outer and an inner part 

 {premier lobe protocerebral and deuxitme lobe protocerebral). Cholodkowsky 

 also (No. 20) observed the segmentation in the brain of Phyllodromia. He, 

 however, considers the optic ganglia as belonging to the third segment of the 

 brain. 



The above considerations incline us to regard the primary cephalic region as 

 being derived from three fused segments. Of these the most anterior would 

 have to be called the true primary cephalic segment. The segment of the brain 

 belonging to it (the protocerebrum) would be the homologue of the Annelidan 

 brain derived from the neural plate. The second cephalic segment which we 

 should have to identify with the antennal segment* would have to be regarded 

 as a post-oral trunk-segment which has shifted forward secondarily {\\ 295), and 

 the third cephalic segment would also have to be regarded in a similar manner, 

 being followed eventually by the hypothetical pre-mandibular segment and then 

 by the mandibular segment. 



Taking into account what has just been said, it must appear remarkable that, 

 so far, observers have been able to find only one pair of coelomic sacs in the 

 primary cephalic region (p. 320). This pair, according to Cholodkowsky, 

 belongs to the antennal segment into the appendages of which it is prolonged. 

 We should have to assume that the pair of primitive segments between these 

 coelomic sacs and those of the mandibular segment have been secondarily 

 suppressed. 



It should be mentioned that the frontal ganglion and the unpaired oesophageal 

 nerve connected with it are independent structures which only secondarily enter 

 into connection with the brain. They owe their origin to an ectodermal 

 invagination which belongs to the anterior wall of the oesophageal depression. 

 This invagination yields the material for the formation of the frontal ganglion 

 and the oesophageal nerves (Heider, No. 38 ; Carriers, No. 13). 



* It should be mentioned that Patten (No. 67) and Carriers (No. 13) 

 reckon the antennae as belonging to the third brain -segment. [Burger, in 

 his work on Chalicodoma (No. II.), based largely on Carriere's notes, claims 

 the antennae as belonging to the deutocerebral segment. A pair of minute 

 evanescent appendages were found by Carriere on the protocerebral and 

 another pair on the tritocerebral segment. It is thus evident, from his post- 

 humous work, that Carriere had ceased to reckon the antennae as belonging 

 to the tritocerebrum. — Ed.] 



