272 THE INVERTEBRATA 



notopodia of the tenth chaetiferous segment, carry these strings of 

 mucus to the cup-shaped organ where they accumulate to form a ball 

 of food which is carried forward in a dorsal groove to the mouth. 



The burrowing Polychaeta 



Arenicola marina (Fig. 189) is the type of a burrowing polychaet 

 and it has a rounded cross-section like an earthworm. In its division 

 of the body into regions, the modification of the parapodia, and the 

 internal anatomy it resembles the tubicolous worms. The prostomium 

 is much reduced, however, without any appendages and there is an 

 eversible pharynx, covered with minute papillae, which is the organ 

 for locomotion through the sand as well as for feeding. In general 

 form it thus resembles an earthworm : the chief obvious difference is 

 the presence of gills and parapodia. It is divided into three regions : 

 the anterior, consisting of the peristomium, an achaetous segment, 

 and six segments which have a notopodium with capillary setae and 

 a neuropodial ridge with chaetae resembling uncini (crotchets); the 

 median, the segments of which have gills in addition; and the pos- 

 terior, in which parapodia and chaetae are entirely lost. 



The body wall consists of the typical circular and longitudinal 

 muscle layers as in Lumbricus, and by their alternating contraction and 

 expansion the peristaltic movements which are characteristic of the 

 earthworm and other burrowing forms are carried out. In Nereis and 

 other surface-living forms progression takes place in two ways, 

 (i) By alternate flexing of the two sides swimming movements are 

 brought about. The longitudinal muscles, which are arranged in four 

 bundles, are much more important than the circular and are capable 

 of rapid contraction. (2) By successive movement of the parapodia 

 crawling movements occur (as in a centipede), the special parapodial 

 muscles coming into action. In tubicolous forms peristalsis occurs, 

 but the longitudinal muscles are even more important than in Nereis 

 for the violent movements of contraction which withdraw the animal 

 into its tube. They form a bulky dorsal mass and resemble the columella 

 muscle of the gasteropod in their action (Fig. 186 C). 



Arenicola is the most convenient polychaet type for dissection 

 and therefore the following details of internal anatomy are given 

 (Fig. 190). In several prominent features it differs from Lumbricus and 

 also from Nereis or Eunice. The body cavity is spacious, it is not en- 

 croached upon by the longitudinal musculature, and the vertical septa 

 which primitively separate the body cavities of the segments have 

 nearly all disappeared. Only the three anterior septa and an indefinite 

 number of the most posterior are preserved. In the greater part of the 

 body the coelom is thus uninterrupted. In its general development 

 the alimentary canal resembles that of the earthworm. The muscular 



