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 1. 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 
