268 



THE ANIMAL KINGDOM 



Strict and thereby elongate the segment, and beneath this are several 

 bands ot longitudinal muscles that can produce shortening and thick- 

 ening. Various oblique fibers may also be present. Between the body 

 wall and the digestive tract is a spacious coelom, divided by thin mus- 

 cular septa between segments into a series of annular (ring-like) cavities. 

 Each of these originates as a pair of lateral cavities lined with a delicate 

 mesodermal peritoneum. The cavities become enlarged until they fill 

 the segment, but the two peritoneal sacs remain intact, lining (1) the 

 body wall on each side, (2) the septa before and behind, and (3) the di- 

 gestive tract between them. Above and below the digestive tract the two 

 membranes meet to form the dorsal and ventral mesenteries. These may 

 persist in the adult, but in most species one or both later disappear. 



1 1 7. Classification of the Phylum 



Po/ychaefes. Most of the marine annelids have eyes, tentacles and 

 palps on the prostomium, and lateral appendages on the body segments. 

 The latter are flaps of the body wall, the parapodia, bearing tufts of 

 many bristles, the chaetae. These annelids are placed in the class Poly- 

 chaeta (Fig. 15.1). 



Most polychaetes live near the shore and on the bottom of shallow 



Figure 15.1. Classes of the phylum Annelida. Polychaeta: Nereis virens, the clam- 

 worm. Oligochaeta: Lumbricus terrestris, the earthworm. (After Lawson, et al.) Hiru- 

 dinea: Hirudo medicinalis, the medicinal leech. (After Hegner.) 



