PHYLUM ANNELIDA 



97 



c) External openings: The mouth opening in the peristomium has already 

 been noted. The last segment contains the anus. On the ventral surface of 

 the fifteenth segment is a pair of conspicuous openings with swollen lips, the ends 

 of the male ducts. The openings of the female ducts are on the fourteenth 

 segment and much smaller than the male openings but not especially difficult 

 to find on large specimens. There are also the openings of the two pairs of 

 seminal receptacles on the ninth and tenth segments, a pair of excretory openings 

 on the ventral surface of each segment, except the first three and the last, and 

 dorsal openings, the dorsal pores, connected with the coelome, but all of these 

 are minute and impossible to find. 



d) Setae: The earthworm, like Nereis, bears setae which are arranged in 

 four double longitudinal rows, two ventral and two lateral. Pass your finger 

 up and down the earthworm and feel the setae as rough projections. With a 

 hand lens make out their arrangement on each segment; the anterior part of 

 the body is the most favorable place to see them. They are used by the earth- 

 worm to prevent slipping. Make a diagram, representing a segment as a ring 

 and show the position of the setae on the ring. 



2. Internal anatomy (Hegner, pp. 168-69). 



a) Structure of the body wall, coelome, mesenteries: The body is covered with 

 an iridescent thin cuticle which is secreted by the epidermis. Strip off a small 

 piece, spread out on a slide in a drop of water, cover, and examine with the high 

 power. It shows striations at right angles to each other, which are the cause of 

 the iridescence, and numerous pores, which are the openings of the gland cells 

 of the epidermis to the exterior. Draw a small portion of the cuticle. 



With a scissors cut through the body wall a little to the left of the median 

 dorsal line from a point beginning about an inch behind the clitellum and extend 

 the cut up to the prostomium. Be careful not to cut through anything but the 

 body wall; if black material oozes out you may know that you are cutting into 

 the intestine and should withdraw your scissors and begin again less deeply. Sepa- 

 rate the edges of the cut and look inside. Observe that the body wall is separated 

 from the intestine, the central dark tube, by a distinct space, the coelome. Ob- 

 serve further that this space is not continuous but is divided up into a series of 

 compartments by delicate white partitions, the septa or mesenteries, which 

 extend from the body wall to the intestine. What is the relation of these septa 

 to the external rings? The coelome of the earthworm thus consists of a longi- 

 tudinal series of chambers, which are paired, that is, one on each side, the intestine 

 being inclosed between the two members of each pair. The inner walls of each 

 pair of coelomic compartments therefore meet above and below the intestine 

 to form dorsal and ventral mesenteries, although these are no longer complete 

 in the adult worm. The anterior and posterior walls of each compartment 

 come in contact with the walls of those in front of and behind it, producing 

 double- walled partitions, the septa already mentioned. The outer wall of the 



