Phylum 5. Annelida. 



The elongate, bilaterally-symmetrical body consists of a number oi' 

 somites or segments, which are separated externally by con- 

 strictions ; the segments resemble each other to a certain extent both 

 in their internal and external structure, although they are never all 

 identical, the first or several anterior segments and the last always 

 differing from the rest. Frequently, too, there are other differences, 

 but, even when these variations are considerable, certain common 

 features are always retained. 



Fig. 128. Annelid seen from the side ; diagram of alimentary canal, of the nervous 

 system, and the segmental organs, m month, a anns, c cerebral ganglion, 6 sub-oesophageal 

 ganglion, s segmental organ. — Orig. 



The body is covered by a thin cuticle. The mouth is close 

 to the front end. The alimentary canal, consisting of several regions, 

 visually traverses the body without convolutions, although not in- 

 frequently it is provided with lateral evaginations ; the anus is at 

 the hind end. The central nervous system (Fig. 129) consists 

 of a paired cerebral ganglion above the anterior end of the 

 digestive tract, and two nerve cords passing from this round the 

 buccal cavity and then running close together below the alimentary 

 canal in the ventral body- wall. In each segment these nerves swell out 

 into a pair of ganglia ; the two ganglia of a segment are united by a 

 longer or shorter commissure. The cords often lie close to one anothei-, 

 or are even fused, in which case the ganglia of each pair are united. 

 By fusion of several consecutive seg-ments, the ganglia are aggregated 

 and even coalesced. From the cerebral and ventral ganglia nerves 

 go to the corresponding segments. As to sense organs, tactile 

 threads (tentacles, etc.) are often present, and also eyes: the latter, 

 which are usually few in number and simple in structure, are 



