130 INVERTEBRATE ANIMALS, 
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 Szpunculus 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 number of species of 
this class have been recorded as occurring in British seas, and 
one of the more characteristic forms is figured above (fig. 57). 
Crass II. ANNELIDA—The Axnelida 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 except at the two ends of 
the body. All the Avwmelida 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. 58) consists of two arches, 
Fig. 58.—Diagrammatic transverse section of a typical Annelide. d@ Dorsal arc ; 
v Ventral arc ; 2 Branchiz or gills ; a@ Dorsal oar; 4 Ventral oar—both carrying 
bristles and a jointed filament. 
termed, from their position, respectively the “dorsal arc” (d), 
and the “ventral arc” (v). Each segment carries a lateral 
process on each side, which are known as the “ foot-tubercles ” 
(darapodia). Each foot-tubercle in turn may consist of an 
upper piece or “ dorsal oar” (a), and a lower piece or “ ventral 
oar” (4), both carrying a tuft of bristles and a soft jointed fila- 
ment (the “ cirrhus”). 
The nervous system consists essentially of a double gang- 
liated chain placed along the ventral surface of the body, and 
traversed in front by the gullet, so that the first ganglion lies 
