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WILLIAM J. DAKIN 



brain of a Copepod — Eucalanus attenuatus (Dana), 

 but Esterly (7) considers these to be optical in function and 

 apparently their structure is quite different from that of the 

 other brain vesicles we have dealt with. 



We would suggest, therefore, that the infra-cerebral vesicles 

 of Peripatus are homologous with the cephalic pits of other 

 Tracheate embryos. In these cases the pits become closed 

 off, the walls become parts of the cerebral ganglion, and the 

 cavities disappear altogether. In Peripatus, on the other 

 hand, the vesicles remain, but they are gradually constricted 

 off from the rest of the supra-oesophageal ganglion. The adult 

 condition in Peripatus is, then, an embryonic stage in the 

 Myriapoda. In the adult it is probable that the infra-cerebral 

 vesicles serve no special function — they are not really ' organs ' 

 at all — they may still be regarded as parts of the supra-oesopha- 

 geal ganglion. Possibly their wall contains a few ganglion cells 

 from which fibres pass into the deeper parts of the brain mass. 

 The occasional presence of bodies within the cavity is interesting, 

 but this suggests nothing more than the ectodermal origin of the 

 cells of which the vesicle is composed, and the tendency for 

 the secretion of a chitinous cuticle. 



In the earlier stages before growth is complete this portion 

 of the supra-oesophageal ganglion retains some of its former 

 power of growth and continues to give rise to cells by division 

 (as observed by Duboscq), but it is not a special organ for this 

 purpose and loses its function in the adult. 



Whether this character, apparently common to the Tracheates, 

 is homologous with the statocysts found occasionally in the 

 worms is another matter — quite impossible as yet to decide. 

 It has been affirmed, however, that in certain Annelids (Lopado- 

 rhynchus) the supra-oesophageal ganglia develop in connexion 

 with ciliated pits, which degenerate somewhat afterwards (13). 

 This is very suggestive. 



