106 MORPHOLOGY OF INVERTEBRATE TYPES 



The anterior five segments form the head which, however, is not 

 externally distinguishable from the following twenty-one seg- 

 ments which form the trunk. The posterior seven segments are 

 modified in the adult leech into the so-called posterior sucker. 

 Externally the segmentation is shown by the excretory pores, 

 but is otherwise hard of recognition owing to the presence of 

 transverse folds of the skin known as rings or annuli. The typi- 

 cal number of annuli for each segment in the Hirudinei is three, 

 but in the medicinal leech we find that the first and second 

 segments are composed of a single annulus each, the third and 

 fourth of two annuli each, the fifth and sixth of three annuli 

 each, the seventh of four annuli, the eighth to twenty-third of 

 five annuli each, the twenty-fourth, twenty-fifth and twenty- 

 sixth of two annuli each, making altogether one hundred and two 

 annuli. The skin of the leech is composed of a cuticle and a 

 single layer of epithelial cells, which we shall call the hypodermis. 

 Inserted between these cells there are many unicellular slime 

 glands the number of which is especially great in the ninth, 

 tenth and eleventh segments where they produce the cocoon 

 covering the eggs during oviposition. These segments repre- 

 sent therefore the clitellum and are equivalent to the clitellum 

 of the earthworm. The skin is the seat of special sense organs 

 in the form of sensory papilla especially well developed in the 

 middle annulus of each segment. On the dorsal surface of the 

 animal in the anterior five segments forming the head, are five 

 pairs of eyes, one pair in each segment; the third and fourth 

 pairs are located in the first annulus, the fifth pair in the second 

 annulus of the corresponding segment. The color of the skin is 

 due to special branched pigment cells situated under the hypo- 

 dermis between the muscles. 



The muscular system is highly developed. Immediately 

 under the hypodermis is a layer of circular muscles. The 

 function of these muscles is to extend the animal. The space 

 between the circular muscles and the internal organs is almost 

 completely occupied by a powerful layer of longitudinal muscles. 



