SEGMENTATION OF THE HEAD 9 



developments from a single ganglionic centre, representing the 

 highly modified annelidan archicerebrum. The tritocerebrum, 

 according to Hanstrom, is the ganglion of the first true head- 

 segment. The arthropod head, on the basis of his theory, is 

 composed of four segments only, viz., those represented by the 

 second antennae, mandibles, maxillae and labium, together with a 

 pre-oral or prostominal region bearing the labrum, eyes and first 

 antennae. The coelomic cavities present in front of the trito- 

 cerebrum are interpreted by him as being of secondary origin and 

 devoid of metameric significance. The first antennae and pre- 

 antennae come under the category of prostomial tentacles and are 

 consequently not regarded as being true appendages. Hanstrom's 

 views are accepted by Snodgrass (1935), and without going into 

 details it needs to be stressed that great importance is given to the 

 general occurrence of corpora pedunculata, or mushroom bodies, as 

 indicating cerebral homologies. These structures constitute the 

 largest and most important association centres in the insectan 

 brain. They are, furthermore, present in all arthropods and are 

 also of common occurrence in the archicerebrum of annelids. 

 Relatively slightly developed in the Polychaete family Hesionidae 

 they become more highly differentiated structures in the families 

 Aphroditidae and Nereidae and ultimately attain their greatest 

 development among the Insecta. The evidence brought forward 

 by Hanstrom leads to the conclusion, at any rate, that the 

 annelidan archicerebrum forms a large part of the arthropodan 

 protocerebrum. Hanstrom also maintains that the optic centres 

 or arthropods have been evolved from homologous tracts located 

 in the optic lobes of the archicerebrum, as seen in the Polychaete 

 family Emiicidae. While these primitive optic centres in the 

 Polychaeta have a very simple structure, they appear to grade into 

 the more elaborate, but still elementaTy, optic centres found in the 

 Branchiopod Crustacea. While the value of Hanstrom's studies 

 is not likely to be disputed, his conclusions appear to be the result 

 of too much stress being laid upon brain structure alone. While 

 it may be granted that evidence of the existence of pre-antennae 

 in insects is scanty, it is difficult to accept his conclusions relative 

 to the deutocerebrum with its undoubted ccelom sacs and well- 



