PRINCIPLES OF ANIMAL BIOLOGY 27 



66. Internal Structure. Handle the specimens with care. They must 

 not be cut up or destroyed except as indicated later in the instructions. 



Find the dorsal side of the animal. Insert the point of the scissors 

 through the body wall a little behind the clitellum near the mid-dorsal 

 line. Be sure that the scissors do not pass into the internal organs. 

 Now cut the body wall backward to the posterior end, then forward to 

 the anterior end, the cut always passing close to the mid-dorsal line. 

 Be especially careful near the anterior end, about the third somite, not to 

 injure the brain. 



Separate the cut edges a little, just behind the clitellum, and note the 

 transverse partitions or septa (singular septum), which divide the body 

 cavity or ccelom into compartments. The ccelom surrounds the digestive 

 tract. Note the relation between the septa and the intersegmental 

 furrows on the surface of the worm. 



Now cut the septa carefully on each side for about an inch. The 

 best instrument for this is the point of a sharp dissecting needle. Lay 

 the worm ventral side down in the dissecting pan and pin the body wall 

 out flat as far as the septa have been cut. Slant the pins outward so as 

 to leave room to work between them. Then with the point of the needle 

 cut or tear the septa forward and backward, putting in pins whenever 

 necessary. When this dissection is completed the septa should have been 

 cut to the same depth on each side. Be careful not to injure any of 

 the internal organs. Remember the general rule in dissection, to cut 

 nothing unless you know what it is and why you cut it. 



Readjust the pins in the anterior region so that they pass through 

 the walls of the fifth, tenth, and fifteenth somites. This will facilitate 

 counting them in locating the organs. Now study the following systems 

 of organs. 



6c. Reproductive System. Male Organs. In somites 9, 11, and 12 

 notice the three pairs of whitish bodies partly covering the alimentary 

 tract. These bodies are the seminal vesicles. In them are located the 

 testes which produce the spermatozoa. 



None of the remaining male organs are visible without careful dis- 

 section. They may be omitted from the study of the dissection but 

 should be studied from a chart and from figures in the text-book ("Princi- 

 ples of Animal Biology," by Shull, La Rue and Ruthven). 



Female Organs. These consist of the paired ovaries in the 13th 

 somite and a pair of oviducts in the 14th. Both organs are small and 

 need not be found, but should be studied from a chart or text figures. 



Close to the septa separating somite 9 from 10 and 10 from 11, are 

 two pairs of small whitish bodies, the seminal receptacles. Mature sper- 

 matozoa received from another worm are stored in these. Be careful 

 in the course of the dissection not to remove or injure the reproductive 

 organs. 



