STUDIES ON LIMULUS. 



EXPLANATION OF PLATE VIII. 



191 



Fig. 3. A sagittal section of Limulus, showing the nerves and principal 

 organs in relief (seen from the right side, somewhat larger than natural size). 



All the prosomatic appendages (ap?'^), except the chelicera {ap}) and chilarium 

 (ap.^ of the right side, are omitted. The operculum {ap.^) and the five gills (a/.''") 

 are represented in the abdominal region. 



All the muscles are also omitted except the fibers running from the occipital 

 ring {oc.r^ to the posterior side of the oesophagus (pe^, the chilarial muscles (7c), 

 the sphincter ani (j.a.), and the levator ani (/.a.). 



The endocranium (endo.), with the occipital ring {per.) and the capsuliginous 

 bar {b.cJ), is seen from the side, and the positions of the abdominal endochon- 

 drites (a.e.^'^^) are indicated. 



The mouth {m.) leads into the oesophagus (oe.), which passes through the 

 brain and forward to the proventriculus {prov.). A constriction, which marks the 

 position of the pyloric valve, separates the proventriculus from the intestine {ini.) 

 which passes posteriorly to the anus {a.). A pair of hepatic ducts {h.d.^ ^""^ ^) 

 enter the intestine opposite the endocranium. 



The heart {ht.), surrounded by the pericardial sinus {p-s.), lies haemal to the 

 intestine. The pericardium (/.) is shown between the heart and the intestine. 

 The ostia {os.'''^^) of the heart and the origins of the four lateral arteries {/.ar.^^) 

 are indicated upon the sides of the heart ; the frontal artery (f-ar.) and the aortic 

 arches (ao.a.), curving down to the brain, arise from the anterior end of the heart ; 

 the superior abdominal artery {s.a.ar.), and the opening of the collateral artery 

 into it, are seen at the posterior extremity of the heart. 



The brain surrounding the oesophagus is seen in side view upon the neural 

 side of the endocranium. The ventral cord {v.c.) passes through the occipital 

 ring {oc.r.) into the abdominal region. The anterior commissure (a.c), with the 

 three rostral nerves (Ja.n.) innervating the rostrum, or labrum {la.), and four of 

 the post-oral commissures are represented. 



The cheliceral nerve («.«.') with the small external pedal branch is shown 

 entire, but the next five neural nerves (w.w.^'') are cut off. The chilarial nerve 

 («.«.^), the opercular nerve («.«.^), and the five branchial nerves («.«.'■") enter 

 their respective appendages, the two former {n.n.^ ^"^^ ^) passing through the 

 occipital ring. 



From the fore-brain the three olfactory nerves {ol.n.) pass anteriorly to the 

 olfactory organ {ol.or.); the median eye nerve {m.ey.n.) passes to the right of the 

 proventriculus {prov.) to the median eyes {m.e.); the lateral eye nerve {l.e.n.) 

 passes forward and is represented as cut off opposite the proventriculus. The 

 lateral nerve (/.«.), or first haemal nerve, is also cut off just beyond the point 

 where it fuses with the second haemal nerve {k.n.'^). The stomodaeal nerve 

 (st.n.) ramifies over the oesophagus and proventriculus. 



The second haemal nerve {k.n.') passes to the anterior extremity of the cara- 

 pace ; its haemal branch is cut off opposite the proventriculus. An intestinal branch 

 (».«.^) arises from near its base and disappears behind the anterior cornu of the 

 endocranium. 



The next three haemal nerves {k.n.^^) are cut off close to the brain, and the 



