106 PERIPHERAL NERVES AND GANGLIA. 



practically nothing is known about the anatomy of these muscles and nerves in 

 other invertebrates. 



In Limulus, the fact that is specially noteworthy is that the six originally 

 vertical components of the branchio-thoracic muscle have been converted into a 

 nearly horizontal, or longitudinal, compound muscle, thereby destroying all 

 correspondence between the metamerism of the neural and haemal surfaces as far 

 as this muscle is concerned. 



The hypo-branchial muscle, by this change in position, gains in effectiveness 

 as a respiratory and flexor muscle, but it would be a mistake, I believe, to accept 



6-dac-- 



FXG. 80. Muscles and distribution of the nerves in the sixth leg of Limulus, from the anterior side, i-cox., 

 Coxopodite, or first joint; 2-bas., basipodite, or second joint; s-isc., ischiopodite, or third joint; 4-car., mer., fused 

 carpopodite and meropodite, or fourth joint; s-pro., propodite, or fifth joint; 6-dac., dactylopodite, or sixth joint; 

 apo., apodeme; bt., brain. 



MUSCLES: 6a and 6b, Plastro-coxal muscles inserted upon anterior side of entocoxite; 6c and 6d, tergo-coxal 

 muscles inserted upon anterior side of entocoxite; e. 2 ~7 , extensors of second to seventh joints;/. 2 " 7, flexors of second 

 to seventh joints; i.m., inter-tergal muscle. 



NERVES: a.e.n., Anterior entocoxal nerve; e.p.n., external pedal nerve; h., haemal branch of integumentary 

 nerve; h.n.e, haemal nerve; i.n. G , intestinal nerve; in.n. 6 , integumentary branch of haemal nerve; i.p.n., internal pedal 

 nerve; l.c.n., lateral cardiac nerve \rn.c.n., median cardiac nerve; m.e.n., median ento-coxal nerve or flabellar nerve ; 

 m.n., mandibular nerve; ., neural branch of integumentary nerve; n.n..6, neural nerve; p., pericardium; p.e.n., 

 posterior ento-coxal nerve; s.c.n.6, segmental cardiac nerves. 



that as a primary cause of the change in position, or as an explanation for the 

 existence of that particular function. The real reason lies deeper, and is to be 

 seen in those changes that have gradually reduced the volume of the haemal organs 

 in the anterior head region. This atrophy or reduction of the haemal surface of 

 the head during the early embryonic periods, inevitably draws the haemal structures 

 of the post-cephalic metameres forward, and is the initial cause of that forward 



