AND EMBRYOLOGY OF LIMULUS. 29 



to the intestine, and becoming intimately united with the neurilemma of the oesophageal 

 nervous collar. They unite at the postei'ior part of that collar, and form a single vessel, 

 which accompanies the abdominal nervous ganglionic chord to its posterior bifurcation, 

 when the vessel again divides. Throughout all this course, the arterial is so closely 

 connected with the nervous system as to be scarcely separable or distinguishable from it. 

 The branches of the arterial or nervous trunks which accompany each other may be 

 defined and studied apart." Afterwards in his "Anatomy of the King Crab," p. 24, 

 Professor Owen thus writes of the arterial system : " On each side the origin of the 

 ' ocellar artery arises one of double the size {ib., e.e), which, diverging from its fellow, 

 curves outward and downward over the fore-part of the intestinal canal (plate 2, A, fig. 1 s) ; 

 it gives off, in this course, a branch which ramifies upon the gizzard, a second to the 

 intestine and liver, the main trunk being continued to the nervous annular centre where it 

 expands, and combines with its fellow of the opposite side to form a sheath for that 

 centre analagous to a ' duramater.' This rather loose sheath is continued along the 

 ganglionic ventral cord, and is prolonged, like a loose neurilemma, upon the nerves 

 sent off therefrom, as it is upon those in connection with the annular centre." ^ 



Our own dissections and microscopic sections have taught us that the brain is 

 enclosed by a thick neurilemma, which is different histologically from the arteries, contain- 

 ing no muscular layer. This layer closely envelops the brain-substance, and there is cer- 

 tainly no space between the brain and its neurilemma for the passage of the blood. Now 

 the two lateral arteries descend from the anterior end of the heart, and open just behind 

 the brain into the space between the oeosophagal ring, and its neurilemma, so that 

 the latter is bathed in blood ; the artery merges into the neurilemma, the sinus 

 being largest on the upper side of the oesophageal ring. On each side of the back 

 of the brain is a large artery for the supply of the brain, but there are no small arterial 

 branches. The whole nervous cord behind the brain, including the ganglionic 

 enlargements, is loosely invested by this neurilemma, the space being very wide 

 between the nervous cord and its loose coat, so that the nervous cord and ganglia 

 •are directly bathed by the blood. This neurilemma (or perineurium) also invests 

 the larger nerves sent off from the ganglia. That the nervous cord fills up but a portion 

 of the space within this outer coat may be seen by reference to plate 6, figs. 12-14. 

 In embedding the portions of the nervous cord to be cut, the interspace is filled with 

 the paraffine preparations. We thus conclude, that while Owen and Milne-Edwards' view 

 is substantially correct, it should be modified somewhat, viz., the blood does not flow 

 around the brain itself, though it may flow around the nerves sent to the simple 

 and compound eyes ; and the nervous system appears to us not to be surrounded by a true 

 artery, but that the thick perineurium becomes a vicarious arterial coat. 



The brain in a Limulus ten inches long, exclusive of the caudal spine, is about 

 six millimetres in diameter ; it is broad and flat above, and on the under side full and 



* Having only the first edition of Owen's Lectures on the remarkable feature of the nervous axis of this Crustacean 



Invertebrates in my library, I can not verify the quotation is its envelopment by an arterial trunk." From this it 



above made from the edition of 1855. In a recent letter would appear that Professor Owen was the first to perceive 



from that distinguished anatomist he quotes a.s follows from that the nervous cord is enveloped by the artery, though 



p. 309. "The sides of the great oeosophageal ring are these organs were afterwards elaborated described and 



united by two transverse commissural bands ; but the most figured by A. Milne-Edwards. 



