STUDIES ON LIMULUS. 187 



EXPLANATION OF PLATE VI. 



>» 

 Fig. I. A view of the entire nervous system of Limulus from the neural side. 

 (About natural size.) 



The carapace is represented as transparent, and all tissues which would obscure 

 the nerves and internal organs are left out of the drawing. The appendages have 

 been removed, but the outlines of the entocoxites {ent.''-^) have been sketched in 

 upon the left side to serve as landmarks and to show the relations of the nerves 

 to the thoracic appendages. The positions of the three sensory knobs are indi- 

 cated by the enclosed areas at their outer extremities. The positions of the 

 abdominal appendages are indicated by the external branchial muscles (e.b.tn?-'^^), 

 the branchial cartilages (b.c?-^^), the tendinous stigmata (^.j.^""), and the abdomi- 

 nal endochondrites (a.^.*""). In the cephalothorax {pros) all the tergo-coxal 

 and plastro-coxal muscles have been dissected away, leaving the endocranium 

 (efido.) with the occipital ring {oc.r.) exposed. Upon the left side of the animal 

 one of the tergo-proplastral muscles (t.p.m.3) is represented, and the branchio- 

 thoracic muscles (b.t.m) are seen extending into the cephalothorax. The longi- 

 tudinal abdominal muscles {/.a.m.) are seen in the abdomen. Upon the right 

 side of the animal all the muscles have been omitted except the haemo-neural 

 muscles {h.n.tn.^'^*). The last two haemo-neural muscles {h.n.m.^T> '^^^ ^4) are 

 represented upon the left side also. The large entapophysis {enta.7 ^^^ ^) of the 

 cephalothorax and the smaller ones {euta.'^'^*) of the abdomen are shown upon 

 the right side. At the base of the telson {U/.) the flexors {t.f.m!) and extensors 

 {t.e.m.h) of the caudal spine are represented as cut off near their insertions. The 

 anal muscles, sphincter ani {s.a.), levator ani {/.a.), and occludor ani {o.a.), and 

 their relations to the anus {a.) are shown in the same region. 



The oesophagus {oe.) runs forward upon the neural side of the endocranium to 

 the proventriculus {prov). From this the intestine (/«/.) passes posteriorly on 

 the haemal side of the endocranium, and emerges upon the posterior side of this 

 structure, whence it may be traced to the anus. 



The brain lies upon the neural side of the endocranium, and the ventral cord 

 {v.c) passes back through the occipital ring {oc.r), haemal to the abdominal 

 endochondrites {a.e.^''^^). All of the neural nerves («.«.'■'*) are cut off, but the 

 haemal nerves (/i.;/.'*'^) upon the left side are represented entire, as are also the 

 nerves from the fore-brain {f.br). 



The first pair of neural nerves {n.n.^) go to the chelicerae. The second to 

 sixth pairs go to the next five thoracic appendages, which are represented by the 

 entocoxites {ent.^-^). The seventh pair of neural nerves («.«.^) go to the chilaria, 

 and the eighth pair («.«.*) to the operculum. The two latter pairs pass through 

 the occipital ring. The neural nerves from the ninth to the thirteenth («.«.9-") 

 arise from the abdominal ganglia and innervate the five pairs of gills. 



From the fore-brain a median olfactory nerve {m.ol.n.) and two lateral ones 

 {l.ol.n. and r.ol.n) pass forward to the olfactory organ ; a median eye nerve 

 {m.ey.n.) passes anteriorly and haemally upon the right of the proventriculus 

 {prov) to the median eyes ; and a pair of lateral eye nerves {l.e.n.) sweep around 

 the outer extremities of the entocoxites {ent.'^-*) to the lateral eye {I.e.) upon the 

 haemal side of the lateral expansion of the carapace. 



