STUDIES ON LIMULUS. 189 



EXPLANATION OF PLATE VII. 



Fig. 2. A portion of the cephalothorax, showing the brain and the relations 

 of the nerves to the tergo- and plastro-coxal muscles, sensory knobs, etc., in the 

 region of the entocoxites (seen from the neural side and magnified about one and 

 one-half times). 



The plastro-coxal muscles {4 »• ^- <=• ^^^d z, 6 ""■ ^- <=• '■ ^"^^ e. etc.) hide the whole of the 

 endocranium except the occipital ring (oc.r.) and the capsuliginous bars (d.cJ). 



The outer portions of the entocoxites {enf.^-'') are indicated with attached tergo- 

 coxal muscles {4 <=- <^- h- ■■ ^^^ ], (, c, d. i, and j. etc.). 



The three areas enclosed by the outer portions of the entocoxites represent the 

 three sensory knobs. The base of one of the chelicerae, with the attached tergo- 

 coxal muscles (^) of that appendage, and the bases of the chilaria, with some of 

 the chilarial muscles (7a and y^), are also shown. 



The four lobes of the nephridia («.^"'), with connecting ducts and nephridial 

 opening i^n.6), are represented lying between the plastro-coxal muscles of the 

 anterior and posterior sides of the second to fifth thoracic appendages. 

 ' The brain lies neural to the plastro-coxal muscles; and the ventral cord (v.c^, 

 with the nerves from the accessory brain, passes out through the occipital 

 ring (oc.r.). The anterior commissure {a.c), with the three rostral nerves {la.n.), is 

 situated in front of the aperture through which the oesophagus passes. Four of 

 the posterior commissures are also seen. 



Upon the left side of the animal the neural nerves {n.n.^'^) of the hind-brain 

 are cut off close to the brain to show the ento-coxal branches underneath. The 

 cheliceral nerves («.«.') turn forward over the fore-brain (/.i>r.). The chilarial 

 {^n.fiJ) and opercular {n.n.'^) nerves proceed from the posterior side of the brain 

 through the occipital ring (oc.r). 



From the fore-brain the three olfactory nerves (ol.ti.) pass forward to the olfac- 

 tory organ (ol.or.); a portion of the median eye nerve {m.ey.n.) is also represented; 

 and the lateral eye nerve {l.e.tt.) sweeps anteriorly around the entocoxites {ent.^) to 

 the lateral eye {I.e.). 



The delicate lateral nerve (/.«.), or first haemal nerve, runs parallel to the 

 lateral eye nerve and at one point fuses with the second haemal nerve (/i.n.^). It 

 is continued posteriorly onto the abdomen. 



The other haemal nerves (/i.n.^'^) radiate from the haemal side of the brain 

 and pass to the sides of the carapace, each one anterior to the appendage of its 

 own metamere. Each divides into a haemal (k.) and a neural branch, and in the 

 four anterior nerves the haemal branch passes haemal to the lateral eye nerve. A 

 small branch turns backward toward the median line and innervates a portion of 

 the epidermis of the haemal side of the carapace. The haemal nerves (A.n.'' ='"'^ *) 

 of the accessory brain pass posteriorly through the occipital ring and out toward 

 the sides of the body. 



Intestinal branches (t.n.^'^ arise from the sixth, seventh, and eighth haemal 

 nerves, but the one from the eighth (t.n.^) is concealed by the overlying chilarial 

 muscles. 



Cardiac branches (s.c.n.^-^) also arise from the sixth, seventh, and eighth haemal 

 nerves ; those from the seventh and eighth {t.nJ and sj f^gg together, and the 



