64 INVERTEBRATE ZOOLOGY 
The digestive organs. The mouth leads into the large pharynx, 
which is composed of an anterior and a posterior portion. With 
sharp scissors cut open the pharynx along the mid-dorsal line 
and note the number and arrangement of the chitinous teeth 
imbedded in its inner surface. Notice the delicate muscles 
passing from it to the body-wall by means of which the pharynx 
can be thrust out of the mouth and drawn back again. They 
are the protractors and the retractors. A pharynx which is thus 
protrusile is called a proboscis. Just back of it is the narrow 
cesophagus with which a pair of small tubular glands communi- 
cates. Back of the cesophagus is the stomach-intestine, which 
extends to the anus. Observe the mesenteries. These are longi- 
tudinal partitions, in structure like the septa, one of which 
attaches the stomach-intestine to the dorsal and the other to 
the ventral body-wall. Press the intestine aside and see the 
ventral mesentery. 
The circulatory system. Nereis has two distinct circulatory 
fluids, the colorless or ccelomic and the red blood fluid. The 
first consists of a plasma in which float amveboid blood cells; it 
circulates freely in the body-cavity or ccelom, being forced by 
the movements of the body from one segment to another through 
small openings in the septa. The red blood consists of a red 
plasma, in which float colorless blood cells, and circulates in 
closed tubes. The most important of these blood vessels are 
two longitudinal tubes, the dorsal and the ventral arteries, 
which lie in the median line, one above and the other below 
the alimentary canal. The former, the dorsal artery, pulsates 
and drives the blood towards the forward end of the body and 
distributes it to lateral segmental arteries. Observe these and 
determine how many there are in each segment; also note the 
capillary network into which the dorsal artery breaks up at its 
anterior end. The dorsal portions of the lateral arteries carry 
the blood to the gills and other organs, whence it collects again 
in the ventral portions of these arteries and is conducted to the 
