120 INVERTEBRATE ANIMALS. 



however, that the Gephyrea are, on the whole, very nearly 

 related to the Holothurians, and it is chiefly from the total 

 absence of any radiate arrangement of the nervous system and 

 internal organs that they appear to be more properly classed 

 with the worms. The Sipunculus or spoon-worm is found 

 burrowing in the sand of many sea-coasts, or inhabiting the 

 cast-away shells of univalve shell-fish. A considerable num- 

 ber of species of this class have been recorded as occurring in 

 European seas, and one of the more characteristic forms is 

 figured above (Fig. 45). 



CLASS II. ANNELIDA (Lat. annulus, a ring). The Annelida 

 or ringed-worms are distinguished from the preceding by the 

 possession of definite segmentation, the body being composed 

 of a number of rings which are all similar to each other ex- 

 cept at the two ends of the body. All the Annelida are more 

 or less worm-like in shape, and in all, except the leeches, the 

 segments are (some or all) provided with lateral appendages 

 which mostly subserve locomotion, but which are never jointed 

 to the body. In the typical Annelida each segment (Fig. 46) 



FIG. 46. Diagrammatic transverse section of a typical Annelide. d Dorsal arc ; v Ventral 

 arc; n Branchiae or gills; a Dorsal oar; & Ventral oar both carrying bristles and a 

 jointed filament 



consists of two arches, termed, from their position, respectively 

 the "dorsal arc" (d), and the "ventral arc" (v). Each seg- 

 ment carries a lateral process on each side, which are known 

 as the "foot-tubercles" (parapodia). Each foot-tubercle in 

 turn may consist of an upper piece or " dorsal oar " (a), and a 

 lower piece or " ventral oar " (), both carrying a tuft of bris- 

 tles and a soft jointed filament. 



The nervous system consists essentially of a double gan- 

 gliated chain placed along the ventral surface of the body, and 



