96 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



b) Posterior end' The last segment differs from the others in that it bears 

 no parapodia. A central opening, the anus, is present, and two long tentacles, 

 or cirri, project from the segment. The presence of the anus marks another 

 advance over the previous forms in which the digestive tract has only one 

 opening. In this respect again Nereis resembles the frog. 



c) Remainder of the body: Note that with the exception of the first and last 

 segments all the segments of the body are alike, not considering minor differences 

 in the size and shape of the parapodia. This similarity of segments is not merely 

 superficial but includes the entire internal anatomy. The same parts will be 

 found repeated in each segment. Nereis is thus an ideal segmented animal. 



d) Parapodium: Cut off one of these with a scissors, mount in water, cover, 

 and examine with the low power of the microscope. The parapodium consists 

 of several lobes, one of which, the gill plate, is very large and leaflike, and serves 

 as a respiratory organ. In some relatives of Nereis it is transformed into a 

 filamentous branched gill. Observe the stiff bristles, setae or chaetae, which 

 project from the parapodium. They are characteristic annelid structures. 



B. THE ANATOMY OF THE EARTHWORM 



i. External anatomy (Hegner, chap, x, pp. 164-68). Obtain a preserved 

 specimen, place in the dissecting pan, and compare with Nereis as to form, sym- 

 metry, segmentation, etc. Are the two animals similar? Distinguish anterior 

 and posterior ends, dorsal (dark-colored, rounded) and ventral (light-colored, 

 flattened) surfaces. Is the head as well developed as in the case of Nereis? 

 Is this related to the habit of life of the earthworm? The head of Nereis is to be 

 regarded as a typical annelid head, while that of the earthworm is degenerate. 

 Are parapodia present? 



a) Head: The first segment is composed, as in Nereis, of a prostomium, the 

 liplike process dorsal to the mouth opening, and the peristomium, the ring 

 surrounding the mouth. The head of the earthworm probably consists of the 

 first four or five segments, as indicated by experiments in regeneration. No 

 eyes or other sense organs are present on the head (this being also associated 

 with the habits of the earthworm) but numerous sensory cells are imbedded in 

 the epidermis of the head and elsewhere. 



b) Clitellum: Anterior to the middle of the worm a number of swollen 

 segments occur, producing a distinct girdle around the body, called the clitellum. 

 The clitellum is a glandular region which secretes the cocoon in which the eggs 

 of the earthworm are laid. On the ventral surface of the clitellum is a pair of 

 thickened ridges, the tubercula pubertatis. How many segments are there in the 

 clitellum? How many segments anterior to the first clitellar segment? Is 

 this number the same in all individuals? Remember that the prostomium is 

 not a segment. 



