278 ANNELIDA 



Internal Structure. The body wall consists of the cuticula, which 

 forms the outer covering, the hypodermis, a single layer of cells which 

 secretes the cuticula, and two layers of muscle fibers, an outer circular 

 and an inner longitudinal layer. In the Hirudinea and Myzostomida the 

 body cavity is filled more or less completely with a vacuolated connective 

 tissue and muscle fibers, similar to the parenchyma of the Plathelminthes. 

 In other annelids a spacious body cavity is present, which is usually 

 divided by the dissepiments into as many separate spaces as there are 

 somites. 



The mouth leads into the pharynx (Fig. 487,5), which in most anne- 

 lids can be thrust out through the mouth to form a proboscis, and 

 is the means by which the 'animal takes its food. An oasophagus is 

 usually distinctly marked and is followed by the intestine, which in most 

 cases is straight and passes to the anus at the hinder end of the body; in 

 many annelids a pair of lateral intestinal pouches is present in each 

 somite and a dorsal and a ventral longitudinal mesentery joins the intestine 

 with the body wall. 



A well-developed system of blood tubes (Fig. 487, 7) is present in all 

 except the lowest annelids, which often carry red blood, the most impor- 

 tant of which are a dorsal longitudinal tube just above the intestine, a 

 ventral tube just beneath it, and transverse tubes placing these in com- 

 munication with each other. The body cavity has also a circulatory fluid 

 of its own which in many annelids is in open connection with the blood 

 tubes. 



The excretory system (Fig. 479, 6) consists typically of a pair of 

 coiled tubes called kidney tubules or nephridia in each somite. Each neph- 

 ridium opens into the body cavity by the ciliated nephrostome at one end 

 and to the outside through a pore in the body wall at the other. Respira- 

 tion is carried on through the entire outer surface of the body. In the 

 Polych&ta, however, special respiratory organs are present in the form of 

 projections of the parapodia or the appendages of the head. 



The nervous system (Fig. 479) consists of paired, segmental, ven- 

 trally situated ganglia and a pair of longitudinal nerves joining them. 

 The anterior ganglionic mass constitutes the brain ; it is dorsal in position, 

 being situated above the pharynx in the prostomium. In their most 

 primitive condition these two longitudinal nerves are separated from each 

 other, but in most annelids they lie close together, forming a single strand. 

 Special sense organs exist in a variety of forms. Tactile organs in the 

 form of the paired appendages of the head and trunk characterize the 

 polychaets and the Myzostomida; in the oligochaets and leeches the entire 

 forward portion of the body is highly sensitive. Eyes are present in 

 polychaets and leeches, and a few of the former also possess lithocysts. 



