120 



How does the Earthworm move ? 



The contraction of the longitudinal muscles shortens 

 the body ; the chaetae then project, and stick into the 

 ground, and thus a firm hold is secured. When the circular 

 muscles contract, the body lengthens out and the chaetae 

 are accordingly withdrawn. 



Arenicola (The Lobworm). 

 A Type of Polychaeta. 



State briefly the External Features of the Lobworm. 



The body has three distinct regions : an anterior region 

 of seven segments, each of which (except the first) has two 

 lateral tufts of bristles ; a middle region of thirteen segments 

 with bristles and also gills or branchiae ; and a thin posterior 

 region, which varies in length and number of segments 

 and which has neither bristles nor gills. The mouth is 

 overarched by the prestomium ; the anus is terminal. 



Describe a typical Parapodium (e.g., of Nereis), and compare 

 with it the Parapodium of Arenicola. 



The parapodia, which function as limbs or (when modified) 

 as swimming organs, are hollow lobed outgrowths of the 

 sides of the body-wall. Typically each parapodium con- 

 sists of two similar halves, a dorsal notopodium and a 

 ventral neuropodium ; and each of these has a bunch of 

 chaetae or bristles and a large needle-like bristle called the 

 aciculum. There is also a dorsal tactile cirrus on the noto- 

 podium, and a similar ventral cirrus on the neuropodium. 

 The chaetae are lodged in setigerous sacs, and can' be 

 pushed out or withdrawn. In many instances the cirri 

 are flattened and leaf-like for swimming ; and are some- 

 times adapted to form gills. 



In Arenicola the parapodium is reduced. The noto- 

 podium is a small process with a tuft of setae ; and the 

 neuropodium is a separate flat and narrow pad bearing 

 a row of minute hooks. 



