latter the sexual opening is marked by two long slender spicules, 

 which may be protruded or retracted. 



5. Nervous system, is difficult to see. It consists of a circum- 

 oesophageal ring from which a number of branches run forward, 

 and two principle trunks run backward in the mid-dorsal and ven- 

 tral lines. 



6. Observe the manner of swimming. Run 50 per cent acetic 

 acid and then 50 per cent alcohol under the cover glass. How does 

 it affect the animals? Test in same way any other small animal. 

 How do you explain the results ? 



X. LUMBRICUS TERRESTRIS (The Earthworm). 



A. EXTERNAL CHARACTERS. 



Place a preserved worm in a dissecting dish, cover with water, and 

 observe :- 



1. General form, color, irridescence. 



2. Anterior and posterior ends? How do they differ? Dorsal 

 and ventral sides ; how distinguished ? Right and left sides ; are they 

 symmetrical ? 



3. Body divided into metameres, or somites by grooves around 

 it. Count the somites. 



4. Between the 29th and 35th somites, a swollen light-colored 

 region, the clitclliim. How many somites does it cover? 



5. The setae, stiff light-colored spines projecting from the sur- 

 face of each somite, and easily felt with the fingers. How many 

 are there on each somite, and how are they arranged? Do they all 

 point in the same direction? Remove a seta, mount it in a drop of 

 water, and examine it under the compound microscope. What is 

 its general shape? Do its ends differ? 



6. The cuticle. Soak an alcoholic specimen in water for a few 

 minutes, and then strip off some of the cuticle. What is its color? 

 Texture? Examine some from the ventral surface and note the 

 cuticular sacs in which the setae are imbedded. What is their 

 shape? Arrangement? Examine the cuticle under the high power 

 and observe the striae crossing one another at right angles. At 

 some of the intersections are pores to allow the escape of secretions 

 of the epidermis. 



/. Apertures, (a) The mouth, in front of the first somite, and 

 below a protuberant lobe, the prostomium, which runs across the 

 first somite on its dorsal surface, (b) The anus, a vertical slit at 

 the end of the last somite. The following apertures are not easily 



39 



