240 THE INVERTEBRATA 



The body wall consists of 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. 



Arenicola (Fig. 179) is the most convenient polychaet type for 

 dissection and therefore the following details of internal anatomy 

 are given. In several prominent features it dilfers from Lumbricus 

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

 encroached 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 pharynx, however, is not well developed, the oesophagus 

 is a thin-walled tube with no such development as the gizzard of the 

 earthworm and it bears only a single pair of coeca. The intestine is 

 the longest part of the gut, the seat of digestion and absorption, and it 

 is invested by a layer of yellow cells. The blood system, which also 

 contains haemoglobin in solution in the plasma, differs slightly from 

 that of Lumbricus : there is a single pair of large hearts, each divided 

 into a ventricle and auricle which connect the important lateral in- 

 testinal vessels from which the branches supplying the gills are derived 

 with the ventral vessel. 



The circulation for that region just behind the heart may be ex- 

 pressed as follows: lateral vessels -> auricle^ ventricle -> ventral vessel 

 -» afferent vessel to body wall and gill -> efferent vessel to subintestinal 

 vessel -> intestinal plexus -> dorsal vessel or lateral vessel. The dorsal 

 vessel does not communicate directly with the heart. 



The segmental organs are, like the gills, only found in the middle 

 region. They are prominent organs lying beneath the oblique muscles, 

 remarkable for the large size of the nephrostome, the dark secretory 

 bag-like portion, the cells of which contain insoluble excreta, and 



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