seen, and must be looked for with a hand-lens, or a dissecting micro- 

 scope. They can often be seen by drying the surface of the worm, 

 and then gently squeezing it, when a small drop will come out of 

 the openings, (c) Sexual apertures, (i) Openings of spermaducts, 

 or vasa def erentia ; two openings surrounded by s\vollen areas on the 

 ventral surface of the I5th somite. From these openings, grooves 

 are often found passing back to the clitellum. (2) Openings of 

 oviducts; two small pores on ventral surface of the I4th somite. 

 (3) Openings of the seminal receptacles or spermatheca, two open- 

 ings on each side between the 9th and loth, and loth and nth 

 somites, in line with the outer row of setae and posterior to them, 

 (d) Nephropores ; openings of the segmental organs or nephridia ; 

 two openings in each somite, one on each side, just dorsal to the 

 ventral pair of setae. ( e ) Dorsal pores ; openings into the body 

 cavity at the anterior end of each somite on the median dorsal line. 

 Dnr^' the anterior and posterior portions of the body to illustrate 

 all that yon Jiave observed. 



B. INTERNAL ANATOMY. 



Extend the worm, ventral side down, in a dissecting pan, and 

 fasten firmly by a pin at each end (the anterior one through the 

 prostomium only ) ; cover with water, and cut open carefully with 

 fine scissors, making the incision along the dorsal side a little to one 

 side of the dorsal median line. Do not cut deep, but merely through 

 the body wall. Carefully cut through the partitions or septa along 

 each side, stretch out the body wall to right and left, and fasten 

 with pins. 



Observe the following structures, dissecting as little as possible 

 to make them out : 



I. GENERAL FEATURES. 



1. Body wall, thick and firm and composed of three layers: (a) 

 A thin cuticle on the outside ; ( b ) a more or less colored layer, the 

 epidermis ; ( c ) a light-colored, and much thicker layer internal to 

 the epidermis, the muscular layer. 



2. Body cavity or coelom, with the digestive cavity passing 

 through it from mouth to anus, and septa or transverse partitions 

 dividing it into as many chambers as there are somites. Each sep- 

 tum passes from the digestive tract to the body wall. \Yhat is the 

 relation of the septa to the external grooves ? 



3. Seminal vesicles, large lobed bodies between the roth and 

 1 5th somites, partly covering the digestive tract. 



40 



