54 IOWA STUDIES IN NATURAL HISTORY 
On the next 7 segments the torus is lateral and ventral to the sete tuft. 
The thoracic segments are provided with shield glands, which are con- 
tinued down the abdomen along the ventral groove. 
The abdomen begins on the ninth segment, and throughout the re- 
mainder of the body the torus is dorsal to the sete tuft, or fascicle. On 
reaching the thorax the median ventral groove bends to the left and is 
continued along the dorsal surface to the head. This groove is ciliated 
and serves to carry the feces out of the tube. The parapodia are not 
well developed. 
The sete fascicles on the thorax are like those of the abdomen, only 
much larger. There are two kinds of sete: the one is shaped like a long, 
slender shaft, the apex being slightly bent and narrowing to a point, and 
it is lightly striated; the other is broader with a sharp angle at its 
maximum width, from which it gradually tapers to a point. The uncini 
in the tori are sharply curved hooks, S-shaped and lightly striated. 
Internal Anatomy 
Literature on the internal anatomy is exceedingly scant. Descriptions 
are confined to external characters. Nereis is usually taken as a type for 
the Polycheta, but it is an errant carnivorous animal and differs from 
the sedentary and herbivorous forms. 
Digestwe System. The mouth is anterior and opens into the buccal 
cavity. There are no jaws. Eversion does not take place in this form. 
The esophagus (pharynx), (Plate V, Fig. 1) is surrounded by a thick 
muscle. It protrudes into the stomach. The stomach is very large, ex- 
tending through segments 2-7. In segments 4-7 the sides are deeply in- 
folded so as to make a series of chambers connected by comparatively 
narrow openings. This region also has numerous small, short ceca. The 
intestine (Plates VI and VII, Figs. 1, 2, 3, 4) is coiled, one full turn to 
three or four segments, or metameres, though this seems to vary. An- 
teriorly there are numerous ceca of all sizes, and these seem to alternate 
with radiating masses of chlorogogen; posteriorly the ceca are fewer and 
smaller, and the chlorogogen is more abundant, completely filling the 
celome. The anus is terminal. 
Septa. These are apparently complete or nearly so in the abdominal 
region, but not in the thorax. Most of them seem merely a layer of 
peritoneum, non-vascular, and fully transparent; but at intervals, more or 
less irregular, there are muscular partitions varying in thickness. There 
is a dorsal longitudinal mesentery suspending the coiled intestine. The 
septa (and mesentery) do not extend through the muscular layer, and it 
is not always easy to correlate the segments with the external rings. The 
septa of the fifth and sixth segments are very vascular. 
Muscular System. There is a uniform, continuous layer of circular 
muscle, vascular, separated from the longitudinal layer by a fairly thick 
layer of connective tissue. The longitudinal layer is divided into three 
longitudinal bands, one dorsal and two ventral. (Plate VI, Fig. 1). The 
sete muscles were not investigated. The pharynx (e@sophagus) is very 
muscular. 
