68 INVERTEBRATE ZOOLOGY 



The animal is segmented externally, i.e., it is made up of 

 a number of somites or metameres, like the crustacean body. 

 Count the somites, beginning with the segment just back of 

 the mouth, which is the first somite. Notice the equivalence 

 of the somites ; they are apparently all very nearly alike. This 

 lack of specialization is always a primitive character in a seg- 

 mented animal and is in sharp contrast to the condition of the 

 somites in most arthropods. Among the arthropods studied, 

 which most nearly resemble the earthworm in this particular ? 



Notice the moist, slimy surface. Moisture is necessary to 

 the animal's existence; this accounts largely for its nocturnal 

 habits. Notice also the red blood vessels through the semi- 

 transparent body-wall. What movement of the blood can you 

 detect ? What are the differences between the dorsal and the 

 ventral surfaces ? Notice the difference between the anterior and 

 the posterior ends. The forward end is the older; the animal 

 grows in length by adding new somites to the hinder end, but 

 the number of somites is practically complete when the young 

 worm emerges from the cocoon. Notice the ventral position of 

 the mouth and the terminal position of the anus ; note also the 

 thickened ring around the body not far from the forward end. 

 This is the clitellum; its function will be explained in speaking 

 of the reproductive organs. The animal is without organs of 

 special sense ; numerous minute tactile sense organs which are 

 sensitive to light and other stimuli are, however, present. These 

 are distributed along the body but are especially abundant 

 toward its anterior end. 



Exercise l. Make a sketch on a scale of 3 of the ventral aspect 

 of the forward end of the animal back to the posterior border 

 of the clitellum. Indicate the somites and number them. 



The body of the animal may be divided into two portions, the 

 prosoma or the primitive head, and the metasoma or the primitive 

 segmented trunk. The prosoma is further subdivided into the 



