40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



embryonic or adult, do the tritocerebral ganglia lie in the labrum, 

 ganglia themselves are free to move, and hence, according to Henry, 

 the tritocerebral ganglia have been displaced posteriorly and have 

 united with the back of the brain in the adult insect. The tritocerebral 

 segment is commonly said to be the segment of the second antennae 

 in the Crustacea. Henry, however, after establishing the labrum as 

 the tritocerebral segment, asserts that this cannot be true, because, as 

 she correctly observes, "these antennae do not occur on the labrum." 



All this interpretation is so at variance with well-known and long- 

 described facts of arthropod embryogeny and comparative anatomy 

 that it creates a suspicion there is something wrong about it. It 

 appears to be supported on a conviction (Henry, 1947) that the 

 arthropods have been evolved from polychaete annelids, and that the 

 eversible proboscis of these worms is the introverted first two trunk 

 segments. Consequently the mouth of the polychaete is said to be 

 apical on the first segment, and this segment becomes the labrum in 

 the arthropods. (And yet, certainly no arthropod has its mouth on 

 the end of the labrum.) 



In conformity with her claim that the polychaete proboscis con- 

 sists of the first two segments introverted, Henry relegates the 

 polychaete prostomium to the "third segment," and denies its homology 

 with the oligochaete prostomium. This, to say the least, creates a 

 curious discrepancy between these two groups of annelids. Since it 

 is assumed that the arthropods have been derived from the Poly- 

 chaeta, the corollary follov»^s that in the arthropods the oculo-antennal 

 part of the head must be the third segment. Henry's evidence for 

 the segmental nature of the polychaete proboscis has been critically 

 examined by DuPorte (1958), who reports that it is inconclusive. 

 The account by Wells (1954) of the structure and mechanism of 

 the proboscis of Arenicola certainly gives no suggestion that the 

 proboscis is anything other than an eversible anterior part of the 

 alimentary canal. 



A very different concept concerning the nature of the labrum is 

 proposed by Butt (1957). From his own embryological work and 

 that of others he has assembled evidence that in many insects of 

 several orders the labrum is formed from a pair of small lateral lobes 

 that come together and fuse before the mouth. Eastham (1930) says 

 there is no doubt of the bifid nature of the labrum as it first appears 

 in the embryo of Pieris rapae, each half of the organ being a hollow 

 extension of the head wall containing preoral mesoderm. Accord- 

 ing to ]\Iellanby (1936), the labrum of Rhodnhis appears definitely 

 to arise as a paired structure, and it is observed by Ando and Okada 



