88 LABORATORY STUDIES IN ZOOLOGY 



of bristles or 6c(w which aid (he woini in crawUng. These struc- 

 tures render the removal of these animals from their burrows very 

 difficult. Determine the relations of the setae to each other, 

 to the metameres, and to the symmetry of the body. 



The body of the earthworm is covered by a delicate trans- 

 parent cuticle secreted by the cells which lie immediately below it. 



Are there any eyespots to be found? While unable to hear, 

 these animals are very sensitive to vibrations conveyed to the 

 body through the ground and also to various forms of chemical 

 stimuli and light. 



In addition to the mouth at the anterior end and the anus in 

 the last segment numerous other external openings are found on 

 the ventral side of the body. The nephridio'pores, openings 

 of the excretory organs, should be located previous to dissection of 

 the specimen. With the exception of a few somites in the 

 cephalic end and the last somite, a pair of these openings occurs 

 in every segment. They are usually situated somewhat diago- 

 nally in front of the outermost setse of the ventral (inner) double 

 row. If these openings cannot be located examine a specimen 

 under the demonstration microscope. 



Carefully examine the ventral surfaces' of segments 14 and 15 

 for the genital apertures which are larger than those of the 

 nephridial tubes and differently located. The openings of the 

 vasa deferentia are upon rounded elevations one on each side of 

 the ventral surface of segment 15. The oviducts open in a 

 similar location upon segment 14 but are not borne upon 

 elevations. 



The food of the earthworm consists of organic matter contained 

 in the soil which is drawn into the mouth by the bulblike pharynx. 

 This is controlled by small muscles radiating from its walls to 

 the body wall. 



In order to study the digestive system the animal should be 

 dissected under water. By means of pins through the two ends 

 fasten a preserved specimen in water, dorsal side up. Because 

 the alimentary canal lies close to the dorsal body wall great care 

 should he used in performing the following dissection. With 

 forceps hold up the body wall at a point about one inch behind 

 the clitellum and with the points of the scissors make an incision 

 a little to the left of the median line. Carefully continue this 

 cut forward to the prostomium. In order to make it possible to 

 pin out the body wall on each side and thus expose the internal 



