56 PHYLUM ANNELIDA 



Exercise 1. Make a sketch on a scale of 3 of the ventral aspect of the 

 forward end of the animal back to the posterior border of the 

 cHtellum. Indicate the somites and number them. 



Exercise 2. Make a similar sketch of the ventral view of the last four 

 somites on a scale of 3. 



Internal Anatomy. Pin a large worm, which has been killed, 

 firmly to the wax of the dissecting pan by a strong pin at each 

 end ; then make an incision with fine, sharp scissors through the 

 integument in the mid-dorsal line from the forward end of the 

 animal to a point back of the clitellum, taking great care not to 

 cut the viscera lying beneath. It will be noticed that the body 

 cavity is divided into compartments, corresponding to the so- 

 mites, by transverse partitions, which are called septa. Holding 

 the cut edge of the body wall with the forceps, cut the septa where 

 they join it ; then spread out and pin down the body wall, using 

 many pins on each side. 



Observe first the large alimentary canal, which passes straight 

 through the animal; also several pairs of conspicuous white 

 bodies a short distance from the anterior end, which are the 

 sperm sacs. If the specimen has been freshly killed, the red blood 

 vessels will also be seen. Study and identify in detail the follow- 

 ing systems of organs. 



The Circulatory System. The earthworm has two circulatory 

 fluids, a red one and a colorless one. The latter consists of a 

 plasma in which float amoeboid blood cells. It is present only 

 in the body cavity and circulates throughout the body, being 

 driven by the movements of the animal from one somite to an- 

 other through small openings in the septa ; it will, of course, not 

 be visible in a dissection. The red blood consists of a red plasma 

 in which float colorless blood cells, and it circulates in a system of 

 closed blood tubes. The most important of these blood vessels are 

 two longitudinal vessels and numerous circular ones. Observe the 

 dorsal longitudinal vessel in the median line, above the alimentary 

 canal. It is contractile and propels the blood toward the head. 

 Push aside the intestine and observe just beneath it the ventral 

 vessel, which runs parallel to the dorsal one. Notice that these 

 vessels break into small branches at their anterior ends. 



