THE SAND WORM 51 



supporting rod, called the aciculum. Find the two acicula. The 

 large dorsal lobe of the notopodium is a respiratory organ, a gill. 

 It contains branching blood vessels which can be easily seen. 

 Attached to its dorsal edge is a slender, vibratile sense organ, the 

 dorsal cirrus. Beneath the gill are two lobes, one bearing bristles, 

 or setae. The neuropodium is made up of two lobes, one of them 

 setae-bearing, beneath which is a ventral cirrus. 



Exercise 5, Draw a parapodium on a scale of 6 and label the parts. 



Internal Anatomy. Make an incision with fine, sharp scissors 

 in the mid-dorsal line of the integument of the anterior third of 

 the animal, taking great care not to injure the viscera which lie 

 beneath. The body cavity, or coelom, will be seen to be divided 

 into compartments corresponding to the somites, by transverse 

 partitions which are called septa. Holding the cut edge of the 

 integument with forceps, cut the septa where they join it, and 

 then spread out and pin down the body wall, using many pins on 

 each side. 



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 em- 

 bedded 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 pro- 

 trusile is called a proboscis. Just back of it is the narrow oesoph- 

 agus with which a pair of small tubular glands communicate. 

 Back of the oesophagus is the stomach intestine, which extends 

 to the anus. Observe the mesenteries. These are longitudinal 

 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 coelomic, and the red blood fluid. The first 

 consists of a plasma in which float amoeboid blood cells ; it cir- 

 culates freely in the body cavity, or coelom, being forced by the 



