NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 99 



postoral commissure. In many cases the dorsal lobes are developed 

 in the embryo from a single pair of generative centers in the ecto- 

 derm, just as are the corresponding lobes of the brain in the Ony- 

 chophora and in some of the Annelida. Considering, however, that 

 the annelid brain, as shown in the larva of Lopadorhynchus, has 

 probably taken its origin from a number of discrete prostomial 

 ganglionic centers corresponding with the sensory organs of the 

 prostomium, we should expect that a more primitive condition in the 

 arthropods would show that the definitive brain of these animals is 

 likewise a composite structure formed by the union of primarily 



Clp^ ^^Prnt iST> 2g7> 3g-p ?^P Lni 



-Md 

 ~ — iMx 



iMx. 

 27Ax 



Ant ^'^-yX )E<^^^ X .Ant 



A -' 



Fig. 43. — Embryonic development of the cephalic appendages and nerve 

 ganglia of a chilopod, Scolopcndra. (From Heymons, 1901.) 



A, anterior end of germ band with rudiments of appendages, including pre- 

 antennal, antennal, mandibular, and maxillary lobes, but no rudiments of post- 

 antennal (intercalary) appendages, though postantennal (tritocerebral) somite 

 marked by a pair of ganglia (IGng). B, same, later stage (antenna removed 

 on left), showing ganglionic pits (gp) of ectoderm from which ganglia are 

 developed. 



Ant, antenna ; Clp, clypeus ; igp, generative pit of optic ganglion ; ^gp, pit 

 of protocerebral ganglion ; jgp, pit of preantennal ganglion ; 4gp, pit of antennal 

 ganglion ; 5gp, pit of tritocerebral ganglion ; 6gp-8gp, pits of mandibular and 

 maxillary ganglia ; iGng, tritocerebral ganglion ; iL, first leg ; Lm, labrum ; 

 Md, mandible ; iMx, sMx, first and second maxillae ; Prnt, preantenna. 



separate ganglia. Various studies on the development of the arthropod 

 brain, in fact, demonstrate its diffuse origin. 



The best-known example of the development of the arthropod brain 

 from diffuse ganglionic centers is that described by Heymons (1901) 

 in Scolopendra. The embryonic cephalic appendages of Scolopendra 

 that correspond with cerebral rudiments include the persistent an- 

 tennae (fig. 43 A, Ant) and a pair of transient preantennae {Prnt), 

 appendages of the postantennal "intercalary," or tritocerebral, somite 

 being absent. The definitive brain of Scolopendra, according to Hey- 

 mons, is formed by the coalescence of an anterior unpaired ganglionic 

 rudiment and five paired rudiments. The unpaired rudiment arises 



