266 



ARACHNIDA XIPHOSURA 



8- 



FIG. 154. A sagittal section of Limulus, 

 seen from the right side, somewhat smaller 

 than natural size. After Patten and Reden- 

 baugh. 



All the prosomatic appendages, except 

 the chelicera (4) and chilarium (33) of the 

 right side, are omitted. The genital oper- 

 culum (32) and the five gills (28) are repre- 

 sented. 



The muscles are omitted except the fibres 

 running from the occipital ring to the pos- 

 terior side of the. oesophagus, the chilarial 

 muscles, the sphincter ani (27), and the 

 levator ani (24). 



The endosternite (34), with the occipital 

 ring and the capsuliginous bar, is seen from 

 the side, and the positions of the abdominal 

 endochondrites (31) are indicated. 



The mouth (1) leads into the oesophagus, 

 which passes through the brain to the 

 proventriculus (12). A constriction, which 

 marks the position of the pyloric valve, 

 separates the proventriculus from the in- 

 testine (23) which passes posteriorly to the 

 anus (26). A pair of hepatic ducts (15) 

 enter the intestine opposite the endo- 

 craniuin. 



The heart (16) surrounded by the peri- 

 cardial sinus lies above the intestine. The 

 pericardium is shown between the heart 

 and the intestine. The ostia (17) of the 

 heart and the origins of the four lateral 

 arteries (19) are indicated ; the frontal 

 artery (13) and the aortic arches (14) 

 curving down to the brain, arise from the 

 anterior end of the heart ; the siiperior 

 abdominal artery and the opening of the 

 collateral artery into it are shown. 



The brain surrounding the oesophagus is 

 seen in side view upon the lieural side of the 

 endosternite (34). The ventral cord (35) 

 passes through the occipital ring into the 

 abdominal region. The anterior commis- 

 sure (3), with the three rostral nerves (2) 

 innervating the rostrum, or labrum, and 

 four of the post-oral commissures, are re- 

 prese^ted. 



The cheliceral nerve with the small 

 external pedal branch is shown entire, but 

 the next five neural nerves are cut off. 

 The chilarial nerve, the opercular nerve, 

 and the five branchial nerves, enter their 

 respective appendages, the two former pass- 

 ing through the occipital ring. 



From the fore-brain the three olfactory 

 nerves (5) pass anteriorly to the olfactory 

 organ ; the median eye-nerve (10) passes to 

 the right of the proventriculus (12) to the 

 median eyes (11) ; the lajteral eye-nerve (7) 

 passes forward and is represented as cut off 

 opposite the proventriculus. The lateral 

 nerve (9) or first haemal nerve is also cut 



