58 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 chitiiious 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 

 oesophagus with which a pair of small tubular glands communi- 

 cates. Back of the oesophagus 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 amoeboid blood cells ; it 

 circulates freely in the body-cavity or coelom, 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 



