6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



cerebral ganglia of the insects not united with the brain ; they are 

 called the tritocerebral ganglia by Henry (1948) and by Young 

 (1959). In most of the Malacostraca, however, the second antennal 

 nerves arise from the back of the brain, which is now termed the 

 tritocerebrum. The terminology here is somewhat confusing, since in 

 the insects the tritocerebral lobes of the brain are the premandibular 

 ganglia themselves united with the primitive brain. In the higher 

 crustaceans it would appear that only the nerves of the second an- 

 tennae have been transposed to the brain, as depicted by Henry 

 (1948) in a series of drawings of the anterior nervous system of 

 an anostracan, a notostracan, an isopod, an amphipod, and a decapod. 



Among the mandibulate arthropods the segmental composition of 

 the definitive head is quite different in different groups. There is 

 one case in which it appears that the embryonic blastocephalon alone 

 becomes the functional head of the adult, and this is seen in the 

 crustacean order Leptostraca. In Nebalia bipes a small head lobe 

 (fig. 2 B) bearing the eyes, the first antennae, and ventrally the 

 labrum projects freely from beneath the rostrum (A). The large 

 second antennae {2 Ant) arise close behind this head lobe but from 

 the region of the gnathal segments, on which the carapace (Cp) has 

 its attachment, and the antennal muscles here take their origins. 



A distinct head lobe bearing the eyes and the first antennae is pres- 

 ent likewise in the anostracan branchiopods, in the Syncarida, and in 

 Malacostraca having a carapace, but in these forms the head always 

 carries the second antennae in addition to the first antennae and the 

 eyes. The best example of this type of head, termed the proto- 

 cephalon, or by German writers the Vorderkopf, is seen in the 

 Anostraca (fig. 2 C, Prtc). A similar but relatively smaller head unit 

 is present in Anaspidacea and in the decapods (D), in the latter con- 

 cealed beneath the rostrum. The muscles of the second antennae, 

 however, as shown by Schmidt (191 5) in Astacus and by Grobben 

 (1919) in a stomatopod, retain their origins on the carapace as in the 

 Leptostraca. In the anostracan (C) the antennal muscles appear to 

 arise on the line between the protocephalon and the mandibular 

 tergum (//), there being no evidence of a second antennal segment 

 contained in the protocephalon. 



The so-called protocephalon, therefore, appears to be the em- 

 bryonic blastocephalon invaded by the second antennae, but it does 

 not include the second antennal segment. Admittedly it seems an 

 improbable assumption that a pair of appendages should migrate 

 from one segment to another. The second antennae, however, are 

 never developed on the embryonic blastocephalon, and pertain to the 



