NUMBER OF SEGMENTS IN THE HEAD 



51 



the mandibles and first and second maxillae arise respectively from 

 the three succeeding segments. 



While the postoral segments and their appendages are readily 

 seen to be four in number, the question arises as to whether the eyes 

 represent the appendages of one or more preoral segments. In this 

 case embryology thus far has not afforded clear, indubitable evi- 

 dence. We are therefore obliged to rely on the number of neuro- 

 meres, or primitive ganglia. In the postoral region of the head, 

 as also in the trunk, a pair of neuromeres 

 correspond to each segment. (See also under 

 Nervous System, and under Embryology.) 

 We therefore turn to the primitive number 

 of neuromeres constituting the procephalic 

 lobes or brain. 



Prom the researches of Patten, Viallanes, 

 and of 'Wheeler, especially of ViallaiiQS, it 

 appears that the brain or supracesophageal 

 ganglion is divided into three primitive seg- 

 ments. (See Nervous System, Brain.) The 

 antennae are innervated from the middle divi- 

 sion or deutocerebrum. Hence the ocular 

 segment, i.e. that bearing the compound and 

 simple eyes, is supposed to represent the first 

 segment of the head. This, however, does 

 not involve the conclusion that the eyes are 

 the homologues of the limbs, however it may 

 be in the Crustacea. 



The second head-segment is the antennal, 

 the antennae being the first pair of true 

 jointed appendages. 



The third segment of the head is very 

 obscurely indicated, and the facts in proof 

 of its existence are scanty and need farther elucidation. 



Viallanes' tritocerebral lobes or division of the brain is situated 

 in a segment found by Wheeler to be intercalated between the an- 

 tennal and mandibular segments. He also detected in Anurida 

 HKirttt'ma, the rudiments of a pair of appendages, smaller than those 

 next to it, and which soon disappear (Fig. 34, tc. ap). He calls this 

 segment the intercalary. 1 Heymons (1895) designates it as the 



1 We may add, while correcting the proofs of this book, that the important sum- 

 mary, by Uzel, of his work on the embryology of Campodea appears in the Zoologi- 

 scher Anzeiger for July 5, 1897. He observes that the premandibular segment in the 

 embryo is very distinct, and that the two projections arising from it persist in the 



FIG. 84. Embryo of Anu- 

 rida muritima : tc. ap, mi- 

 nute temporary appendage of 

 the tritocerebral segment, the 

 premandibular appendage ; at, 

 antenna; md, mandible; nix 1 , 

 first maxilla ; v/u' 2 , second rnax- 

 illa ; f' J -p 3 , thoracic ; up 1 , up*, 

 abdominal appendages ; an, 

 anus. After Wheeler. 



