OUTLINE OF THE ARACHNID THEORY. 



Similar segmental sense organs are seen in Limulus, but farther removed from 

 the bases of the appendages. The one that develops into the so-called "dorsal 

 organ" (auditory pit of vertebrates) lies opposite the fourth appendage. (Figs. 

 140-153, 5.0.*) Later it becomes greatly enlarged and is a conspicuous feature 

 on the heemal margin of the thorax till after the last moult of the trilobite stage. 

 At the height of its development, it is a disc-shaped thickening, slightly pigmented 



and sensory in appearance. (Fig. 131.) 

 The four remaining pits (Fig. 140) are 

 very faint and transitory, although in the 

 corresponding regions of the adult, there 

 are patches, or knobs of skin that are highly 

 sensitive and richly supplied with nerves. 

 The thoracic segmental sense organs of 

 Limulus and the scorpion lie nearly in line 

 with the cephalic sense organs, and are 

 probably serially homologous with them. 



The Diencephalon and the Mesencep- 

 halon. — We may recognize two groups of 

 thoracic neuromeres, the diencephalon and 

 mesencephalon, approximately correspond- 

 ing with the external divisions of the thorax. 

 The diencephalon, or tween-brain, 

 consists of the first one, or two or three, 

 neuromeres that surround the oesophagus. 

 It includes the large, lateral stomodaeal 

 ganglia that are attached to the median wall 

 of the cheliceral neuromere, but which arise 

 as thickenings, or evaginations, from the 

 side walls of the oesophagus. These neu- 

 romeres contain the swallowing center and 

 an important center for all the taste organs 

 of the more posterior thoracic appendages. 

 (Fig. 114.) 



The enlargement and closer union of the thoracic neuromeres, and the back- 

 ward overgrowth of the rostrum and the optic ganglia, ultimately lead to the 

 closure of the mouth. After it closes, the inner end of the stomodasum persists 

 in vertebrates as the epithelium of the saccus vasculosus, the passageway between 

 the circum-oesophageal neuromeres becomes the infundibulum, and the stomo- 

 dseal ganglia, arising from its deeper side walls, the lobi inferiori. (Figs. 43 and 

 44.) The last position occupied by the arachnid mouth may be identified in 



Pig. 13. — Bunodes lunula. Restoration from 

 numerous specimen in the author's collection 

 obtained from the island of Oesel, Russia. Photo- 

 graph of enlarged plaster model by the author. 

 X about 2. 



