98 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



compartment is fused with the inside of the body wall, forming a parietal peri- 

 toneum, while the inner wall is similarly attached to the intestine, forming a 

 visceral peritoneum. The relations therefore of the coelome and its linings are 

 identical with those that we found in the frog, except that the segmental septa 

 are absent in the latter. 



Cut through the septa so that you can lay out the body wall. In this opera- 

 tion it is best to stick a pin through the body wall on the left side; then lift up 

 the other side of the cut and cut through the septa and pin out the right side; 

 then move forward a short distance and repeat. Do not pull the wall apart 

 but cut the septa with a fine scissors. Stick the pins in obliquely and firmly 

 so that they will not get in your way and will not come out. Open up the body 

 to the prostomium. Note the iridescent membrane, the peritoneum, which lines 

 the body wall. Cover the animal completely with water. 



Before beginning a detailed dissection of the various systems, the following 

 conspicuous structures should be identified. The center of the body is occupied 

 by the long brownish intestine. In the tenth, eleventh, and twelfth segments 

 are pairs of large white bodies. These are the seminal vesicles, part of the male 

 reproductive system. In the ninth and tenth segments are pairs of small white 

 spherical bodies, the seminal receptacles, part of the female reproductive system. 

 Loose white objects on each side of the intestine along its entire length are the 

 excretory organs, or nephridia. 



b) The circulatory system: This system is difficult and unsatisfactory to 

 make out in dissection owing to the small size of the vessels, and the student is 

 referred to Hegner (pp. 172-75) for details. The following parts should be noted: 



(1) The dorsal blood vessel: This is the dark brownish line running along the 

 median dorsal surface of the intestine. In many specimens the two pairs of 

 branches which it receives in each segment from the wall of the intestine can be 

 seen. Its branches from the body wall (torn off, of course) may also be found 

 sometimes. Look for these. 



(2) The hearts: Remove the seminal vesicles from the left side. Note that 

 in the region where they were located and for some distance anterior to this the 

 septa are stronger and more conspicuous than elsewhere, projecting out from 

 the intestine like wings. Look in the eleventh segment, between the places 

 occupied by the second and third seminal vesicles, for a pair of stout tubes 

 arising from the dorsal vessel and extending ventrally. They are often dark- 

 colored because of contained blood. Then look in each segment forward from 

 this up to the sixth for a similar pair of tubes, grasping the winglike septa with 

 a forceps and pulling them backward. These branches of the dorsal vessel, 

 found in the seventh to the eleventh segments, are contractile tubes, inappro- 

 priately called hearts. The first pair is quite small. More than five pairs are 

 sometimes found. Follow the dorsal vessel forward as far as you can beyond the 

 region of the hearts. 



