No. I.] 



STUDIES ON LIMULUS. 



115 



upon the anterior side of the proctodaeum, close to the anus. 

 These serve to draw the anus forward, and by elongating the 

 anal slit possibly assist the sphincter ani in its function. A 

 pair of band-shaped muscles (Pis. VI, VIII, and IX, Figs, i, 

 3-5, La.), or levator ani, are in- 

 serted upon the sides of the anus, 

 and pass haemally and outward 

 to the haemal side of the cara- 

 pace. They serve to open the 

 anus. 



The CJieliceral Muscles (Text- 

 fig, i). — In the chelicerae the 

 first joint is moved by four mus- 

 cles (Text-fig. I, e/,fJ, and l.mJ). 

 Lankester regarded these as one 

 muscle and called it the tergo- 

 coxal muscle of the chelicera. 

 They have their origin on the 

 haemal side of the carapace, de- 

 scend vertically on the median 

 side of the anterior cornua of the 

 endocranium, and are inserted 

 upon the base of the first joint 

 of the appendage. The flexor fig. 

 {^f.') is inserted upon the inner 

 margin, the extensor {e.') upon 

 the outer margin, and a small 

 muscle {l.niJ) upon the sides 

 midway between the flexor and 

 extensor muscles. In these de- 

 scriptions the appendages are 

 supposed to be revolved out- 

 ward, so as to lie at right angles 

 to the median line and parallel to the other appendages. The 

 flexor and extensor muscles act in opposition to each other 

 and give to the whole appendage a movement toward or away 

 from the mouth. The small lateral muscles give it a slight 

 lateral movement. 



Diagram showing muscles and dis- 

 tribution of nerves in clielicera of adult 

 Limulus, from tlie anterior side (natural 

 size). 7, 2, and j, first, second, and third 

 joints of chelicera; <?.'"3, extensors of first, 

 second, and third joints, respectively ; fy^, 

 flexors of first, second, and third joints, re- 

 spectively; l.m..^, lateral muscles of first 

 joint; «.',y.', and /.;«.^ constitute the tergo- 

 coxal muscles; Ji.c.., haemal side of the 

 carapace; br., fore-brain; «.«.', neural 

 nerve or cheliceral nerve; i.p.n., internal 

 pedal nerve ; c.fi.n., external pedal nerve; 

 //.«.', haemal nerve of cheliceral neuromere 

 or lateral nerve. 



