NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS lOI 



fact that the commissures are not formed until after the respective 

 ganglia have taken a preoral position. This proposed explanation, 

 however, is merely the statement of a fact that can as well be taken as 

 evidence that the ganglia themselves are morphologically preoral. 



In the Diplopoda the embryonic rudiments of the nervous system 

 appear to be completely double, for no median ganglionic center has 

 been observed corresponding with the "archicerebral" rudiment 

 described by Heymons in Scolopendra. Preantennal appendages are 

 absent so far as known, and the tritocerebral somite always lacks 

 appendages, as in the Chilopoda. According to Robinson (1907) the 

 nervous system of a i6-day-old embryo of Archispirostreptus consists 

 of a pair of trilobed "archicerebral" rudiments situated before the 

 mouth (fig. 44 E, Arc), and of two ganglionated nerve cords pro- 

 ceeding posteriorly from the latter around the stomodaeum. The 

 first ganglia of the cords (AnfGng), which are distinctly postoral, 

 Robinson claims are the antennal ganglia. The next pair, she says, 

 are the ganglia of the tritocerebral somite (TcrGiig), which has no 

 appendages, and the next pair {MdGng) belong to the mandibles. 

 At a later stage (F), just before hatching, the "antennal ganglia" 

 {AntGng) , to which Robinson says the tritocerebral ganglia are now 

 joined, still lie behind the mouth and are approximated to the median 

 line. It is very surprising, however, that antennal ganglia should be 

 postoral at such a late stage of development, and not yet united with 

 the protocerebrum, so much so, in fact, that the relation of these 

 alleged "antennal" ganglia to the antennae becomes questionable. 

 Robinson gives no evidence of any nerve connection between the 

 ganglia and the antennae {Ant) ; hence we might suspect that the 

 antennae are innervated from the posterior ganglia of the "archi- 

 cerebral" groups {Arc), and that the first postoral ganglia are the 

 tritocerebral ganglia. 



Heymons (1897) gives a brief description of the embryo of 

 Glomeris (fig. 44 C), in which the antennae {Ant) appear as adoral 

 appendages of the cephalic lobes {Pre), whence presumably they 

 derive their innervation. 



A more detailed account of the cephalic and cerebral segmentation 

 of a diplopod is given by Pflugfelder (1932 a) for Platyrrhacns 

 amauros, but it only adds to the difficulties of understanding the 

 development and morphology of the diplopod head. According to 

 Pflugfelder, the protocerebral and deutocerebral elements of the brain 

 appear on the surface of the young embryo of Platyrrhacns as a 

 single pair of preoral cephalic lobes (fig. 44 A, Pre). Just behind 

 the mouth is the somite of the antennae {Ant), and the latter is 



