5i6 J. PERCY MOORE. [Vol. X. 



wave of contraction passes along the heart walls, push the 

 blood along, and then return without offering much resistance, 

 and complete another oscillation before coming to rest. As 

 the wave of contraction passes forward they close the lumen of 

 the heart more effectually than would be possible for the mus- 

 cular walls alone, and add greatly to the efficiency of the organ. 

 The dorsal vessel of the head is very much smaller than the 

 heart, and as the posterior portion is also elastic and somewhat 

 contractile, the flow of blood is doubtless nearly or quite con- 

 tinuous, although the heart contracts intermittently. From it 

 three pairs of conspicuous but delicate lateral trunks arise, cor- 

 responding to the three posterior cephalic annuli, and wind 

 among the muscles around the pharynx to join the sub-neural 

 vessel. The most anterior of these three blood arches is 

 closely associated with the circum-oesophageal nerve connec- 

 tive. Finally the dorsal vessel terminates, after passing be- 

 neath the supra-oesophageal ganglion, in a pair of trunks which 

 arch through the lips close to the salivary gland, and meet 

 ventrally to constitute the supra-neural blood vessel (Figs, i 

 and 17), which is soon increased by the accession of the sec- 

 ond, third, and fourth pairs of cephalic blood arches (Fig. 16), 

 and, in the first body somite, by the fifth pair of arches. 

 Throughout its entire length the supra-neural vessel lies in 

 contact with the dorsal side of the nerve cord, and passes be- 

 tween the ganglia of each pair in a deep groove (Figs, i, 7, 

 II, etc). In the seventh somite it gives off a pair of large 

 ovarian vascular arches, which empty into the dorsal enlarge- 

 ment of the peri-enteric sinus (Fig. i). The supra-neural ves- 

 sel terminates in the tenth somite in a pair of large trunks, 

 which arch around the intestine, and pass forward to empty 

 into the dorsal region of the peri-enteric sinus, thus forming 

 the beginning of the dorsal enlargement, which here receives 

 the dorsal ends of the plexus of blood passages (Figs, i and 5). 

 Of the seven pairs of lateral arches mentioned, a labial and 

 three cephalic pairs, in the pharyngeal region, arise from the 

 anterior prolongation of the heart ; an oesophageal pair, which 

 owing to their great length are often looped into the succeed- 

 ing somite, arise from the anterior end of the heart itself ; a 



