42 IOWA STUDIES IN NATURAL HISTORY 
Among the Polycheta, one branch has a head consisting of 
a prostomium and a peristomium. The former is a lobe over- 
hanging the mouth and frequently bears paired eyes, tentacles, 
and palpi. The latter is the first complete ring and usually 
bears cirri. The second branch of Polycheta has a peristomium 
which is frequently notched. It completely hides the prosto- 
mium, which becomes an insignificant organ. The tentacles are 
reduced, but the palpi become greatly developed. In neither 
branch do the prostomium and peristomium bear parapodia. 
The external segmentation affects the internal structure by 
dividing the celome into somites by means fof septa. In bur- 
rowing and tubicolous forms the septa are frequently incom- 
pletely developed, or even absent; and when the body is less 
distinctly segmented externally, it varies greatly in diameter 
during movement. The alimentary system presents certain 
modifications of a systematic value. It consists of mouth, buceal 
cavity, pharynx, esophagus, digestive glands, stomach, intestine, 
rectum, and anus. In the Nereidiformia the pharynx is pro- 
trusible in part, forming a proboscis which is worked by the 
pharyngeal muscles. Those worms having a proboscis also 
possess jaws and numerous denticles. 
The circulatory or vascular system is well developed. This 
consists of a dorsal and of a non-contractile ventral vessel, ex- 
tending along the whole length of the body and giving off 
paired segmentally-arranged vessels, which pass to the intestinal 
wall and to the body wall. This system of vessels in the major- 
ity of cases contains a respiratory fluid colored red by hemo- 
globin in solution. The blood flows anteriorly in the dorsal 
vessel and posteriorly in the ventral vessel. 
The nervous system consists of a dorsal cerebral ganglion, or 
‘‘brain,’’ connected by cireum-cesophageal commissures with 
the anterior end of a ventral chain of ganglia. The cireum- 
cesophageal commissures spring from the outer corner of the 
brain, and from each arises a nerve which leads to the head 
sense organs. The first ventral ganglion lies in the third seg- 
ment and represents at least two ganglion-pairs fused together. 
Special respiratory organs are present in the Polycheta in 
the form of projections of the parapodia or the appendages of 
the head. 
