SEGMENTATION OF HEAD. 323 



by the coelomic sacks in other groups fails us, for the meso- 

 blastic somites though segmentally divided contain no coelomic 

 cavities. Among the Malacostraca the supraoesophageal 

 ganglionic mass is laid down in Crangon (according to Kingsley) 

 as three consecutive pairs of ganglia, in addition to the optic 

 ganglia, which lie in front of and external to the first pair. Of 

 these three pairs the anterior is preoral, the others originally 

 postoral. The second and third pairs of ganglia are those of the 

 first and second antennae respectively. The development of the 

 corresponding parts in Astacus appears to present no essential 

 difference. From these facts, and from the similarity of the 

 shapes of the mandibles and the positions of the paired eyes 

 in Crustacea and Insects, we are led to regard the segments 

 bearing the two pairs of antennae of Crustacea as homologous 

 with the antennal and premandibular segments of insects, 

 the appendages of the latter attaining in the Crustacea a full 

 development which is denied them in Insecta. The mandibular 

 and two maxillary segments of the Crustacea correspond with 

 the mandibular, maxillular and first maxillary segments of 

 Insects, the first thoracic appendages finding their homologue 

 in the labium. 



In the lower Crustacea (e.g. Daphnia, Fig. 250), the ganglion 

 of the second antennary (premandibular) segment retains a 

 postoral position throughout life. 



Turning now to the Arachnida (including the Merostomata) 

 we find that the chelicerae and the six segments of the cephalo- 

 thorax posterior to them (the segmental significance of the 

 chilaria has been demonstrated by the researches of Kishinouye 

 and Brauer) are represented in development by coelomic sacks, 

 neuromeres and appendages. In front of the cheliceral neuro- 

 mere is the large paired ganglionic mass of the head lobes. 

 No distinct mesoblastic somite anterior to the cheliceran is 

 found in Limulus or Scorpio, but among the true spiders evi- 

 dence of such sacks has been obtained by Kishinouye and others. 

 Indications of paired appendages in front of the chelicerae 

 have been recognized by some observers in the paired origin 

 of the " labrum " in scorpions and spiders, and in prominences 

 formed in relation with the folding in of the brain of spiders, 

 but in neither case is the evidence conclusive. 



In most Arachnids the cheliceral neuromere fuses with the 



