98 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



sacs of Platyrrliacus should represent the preantennal sacs of Scolo- 

 pendra, Carausius, and Hemhnysis, though there are in the diplopod, 

 as in the crustacean, no corresponding appendage rudiments. While, 

 in most cases observed, the preantennal mesoderm is a part of the 

 general mesoderm, the preantennal mesoderm of Henmnysis is said 

 by Manton (1928) to have an independent origin from the germ 

 band just behind the optic lobes. When the arms of the V-shaped 

 germ band of Heiiiimysis later come together, the preantennal meso- 

 derm rudiments are approximated immediately before the mouth. In 

 their growth, Manton says, they extend posteriorly and embrace the 

 lateral and dorsal walls of the stomodaeum, their cavities entirely 

 disappear, and their walls give rise to a part of the stomodaeal 

 ("stomach") muscles, and to the cephalic aorta. 



Coelomic sacs of the labral region of the embryonic head were 

 first described by Wiesmann (1926) in the stick insect, Carausius 

 morosus, and have since been observed by Mellanby (1936) in the 

 hemipteron Rhodnius, and by Roonwal (1937) in a grasshopper, 

 Locusta migratoria. Pflugfelder (1932a) describes in the diplopod 

 Platyrrhacus a pair of mesodermal cavities in the "clypeus" (fig. 44 D, 

 Clp), but since these cavities lie immediately before the mouth, they 

 evidently correspond with those called "labral" in the insects. In both 

 Locusta (fig. 41 A) and Carausius (C) the head mesoderm extends 

 into the labrum (LinMsd) anterior to the stomodaeum (Stom), and 

 the cavities formed in it are thus literally preoral in position (fig. 42 C, 

 LmCS) ; the mesal walls of the labral sacs of Carausius are united 

 before the mouth (D). In Locusta, Roonwal says, the labral and 

 stomodaeal mesoderm is loosely continuous prior to the appearance of 

 the labral cavities (fig. 41 A), but when the sacs are formed the 

 latter are independent structures (B, LmCS). After the disap- 

 pearance of the cavities the coelomic cells remain as two bodies of 

 mesoderm that suggest similar mesodermal masses found in the 

 labrum of certain other insects in which corresponding cavities are 

 not known to occur. 



The definitive brain of the mandibulate arthropods consists of an 

 anterior bilobed part, including the protocerebrum and the deuto- 

 cerebrum, which innervate respectively the eyes and the first antennae, 

 and of a pair of posterior lobes, the tritocerebrum, which innervate 

 the second antennae when these appendages are present. The proto- 

 deutocerebral lobes are always united above the stomodaeum, and 

 thus appear to belong to the prostomial part of the head ; the trito- 

 cerebral lobes, on the other hand, are unquestionably derived from 

 the postoral somite of the second antennae, and are connected by a 



